UNITED STATES OF AMERICA
*" .  .
FOUNDED  1836
WASHINGTON, D. C.
OPO  16—67244-1 
 
 
AN
            INAUGURAL ESSAY,
                 ON THE

               EYE,
                   AND

            ON  VISION.
                SUBMITTED
         TO THE EXAMINATION OF
                   THE
REV. JOHN ANDREWS, D. D. PROVOST, (Pro Tern.)
                   THE
    TRUSTEES AND MEDICAL PROFESSORS
                 OF THE
       UNIVERSITY OF PENNSYLVANIA,
          On the 5th day of June, 1805.
 FOR THE DEGREE OF DOCTOR OF MEDICINE-  ~
                                  / ^y$*X^
          —_____™     t'  /—~X/:/
          =====     >   ■   . y]
       By elisha de butts,  V >/ U.
OF MARTLAND.
X?',
HONORART MEMBER OF THE PHILADELPHIA MEDICAL SOCIETfi^ / ,
" Sit lux....et lux fuit."
         PHILADELPHIA:
PRINTED FOR THE AUTHOR, BY JOHN H. OSWALD.
1805. 
 
TO
   DOCTOR SAMUEL DE BUTTS,

           MOUNT-WELBY,  MARYLAND.


  Dear Sir,

         x\.LTHOUGH fully conscious of the imperfection
of the following Essay,  yet impelled by motives of gratitude
and affection," I must beg leave to offer this small tribute of my
respect, to you, from whom I have received so many proofs of
friendship; and under whose auspices I received the first prin-
ciples of that science to which  my future life is to be devoted.

     In  this, my  first literary  attempt, you will no doubt dis-
cover many errors ; for which, in addition to the difficulty of
the subject, I may justly plead indisposition ; and I can now on-
ly wish  that it were more worthy of your attention.  But inade-
quate as it may prove, to withstand the test of critical disquisi-
tion, I do not hesitate to dedicate it to you, as the only method
by which I can, at present,  acknowledge the sentiments of
high respect for your character, which exist in the breast of

           Dear  Sir,

                  Your affectionate and

                             Very humble  servant,

                                        THE AUTHOR. 
TO
DOCTOR  CASPAR WISTAR,

         ADJUNCT PROFESSOR
ANATOMY,

   IN THE

                                [ft

                          < noi3JSTJ»
UNIVERSITY OF  PENNSYLVANIA,*
              THIS  ESS AT,   •

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«jof»r  sea  i ; ,i jiqinfi .2<Friend,, and J  3/ii lo mob*iw srii 1«
aisle ;.?:■■   j r X -:»./« >i''.oqrtii 3.           oiJo^ftei kfeci' 'twr:
wi/zcs!-.,       ; ,iii X «it   Very lwmble servant,  - ^ ^

L 3^J£n Xd iioi.b'^,^q ^ ■-, h  THE  AUTHOR.
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INTRODUCTION.
    a EW subjects, in the animal ceconomy, are more inter-
esting, than that which I am now going to consider.  The u-
tility of a correct knowledge of the structure of the eye, and
phenomena of vision, is evident to every one, without demon-
stration ; and in a retrospective view of the science of optics,
in her most infantile 6tate, we find the genius of her votaries,
in a great measure, employed in attempting to elucidate this
subject.   But the state of science in general, among the an-
tients, was too imperfect,  to allow them to form correct opi-
nions, respecting phenomena so intimately connected with the
nature and properties of light, the investigation of which, re-
quired a greater extent of knowledge, than they were in pos-
session of. To more modern philosophers,  therefore, and par-
ticularly, to Sir Isaac Newton,  are we indebted, for the com-
pletest investigation, and  most important discoveries, relative
to the nature of light, and consequently,  for our most correct
ideas of the manner in  which  vision is performed.   Whether
we view the organ of vision, in a phisiological or optical light,
we have, on either  hand,  an impressive and illustrative proof
of the wisdom of the Creator,  and an ample fund for serious
and useful reflection.  The vast importance of a correct state
of vision,  in the most trifling affairs of life ; and the pleasure
which the mind  receives, in contemplating the infinite variety
of sublime and beautiful objects, presented to it by nature and
art, through the medium of sight, calls  forth our most lively
                            B 
VI
gratitude and admiration, to those Philosophers, whose labours
have instructed us in that species of knowledge, by which we
are enabled, literally, to give sight to the blind, and to  dispense
happiness to our fellow-creatures, by rescuing them from one
of the greatest evils to which  human nature is liable.

   In attempting a treatise on this subject,  when so many  ce-
lebrated  works are already written on it, by distinguished  au-
thors, I feel myself perhaps justly  liable to the imputation of
temerity: but vision was my  choice ; and  not until I  began
to pay an undivided attention to it,  did I find myself involv-
ed in difficulties, when it was too late to recede.   This,  then,
I  must plead as my excuse.   The want of plates would  ne-
cessarily render the  following sheets imperfect,  if they were
intended to convey information to those unacquainted with the
subject.  But when I consider the eminent talents of  the gen-
tlemen to whose judgment I must submit them,  and whose
good  opinion I am most  anxious to acquire, every  regret I
should otherwise feel, for this deficiency, is entirely obviated. 
SECTION I.
OF  THE  EYE.
      As,  in a description of the eye,  in order to illustrate
a theory of vision, a circumstantial detail  of its external parts,
is not generally esteemed necessary, I will not attempt it here.

  The eye is situated in a bony cavity of the  head,  formed by
the union of seven bones of the face and cranium,  called its
orbit.  This cavity is  irregular in its figure ; but is well sup-
plied with fat and cellular substance, in order  to be completely
adapted to the form of its contents.  In this situation, the eye
is firmly  retained by  its  muscles, nerves, blood-vessels,  &c
and is defended from external  injuries, by the supercilia,  pal-
pebral and lachrymal passages.  When it is detached from the
above cavity,  it appears to be  almost  spherical ;  but, after the
fat  and other appendages are carefully removed, it is found to
be flattened,  posteriorly ; and is usually called, by anatomists,
the ball,  or globe of the eye.   Upon an attentive examination,
it appears to consist of three membranes,  or coats, placed con-
centrically,  within each  other, containing transparent  fluids.
The first, and most external coat,  is called sclerotica.   At it.
anterior part, it forms the transparent cornea, and over every
other part of its surface,  it exhibits a white  cartilaginous ap-
pearance, easily retaining its primitive shape, after being de-
prived of all its contents.   To this coat, which is stronger than
any of the others, are attached six muscles, used in rolling tne
eye  in different necessary directions, the tendons  of  whirl.
spread over part  of its surface, and contribute, not a hulc, to 
                        <.  8   ^
its strength. The sclerotic coat seems to be formed by the ex-
pansion of the inner plate of the dura mater of the optic nerve;
the outer plate forming the periosteum of the orbit.*  The se-
cond coat, or the choroides, originates from the white cellular
circle terminating the substance of  the optic nerve t; and ex-
panding, concentrically, within the  sclerotica, is connected to
it by vessels and cellular membrane. But at the circumference
of  the  cornea, this  cellular substance  becomes  much more
dense, attaching  the coats very firmly to each other,  and is
then called the ciliary band. The choroides, at this place, gives
out, from every part of the circumference, certain radii, inter-
intermingled with vessels, which, terminating at equal distan-
ces from  the centre, leave a  small hole, called the pupil, to
admit the rays of light.  Round the pupil,  and connected to
the above radii, are certain fibres,  disposed  in the form of  a
ring,  which act as a  sphincter muscle, \ the former dilating,
the latter contracting the pupil.  The anterior surface of this
part of the choroides, is termed the iris.   The posterior, from
its uniform resemblance, in colour, to a ripe  grape, the uvea;
but this circumstance alone can make any difference between
them.  The iris varies in colour, in different people, and be-
fore those ages of the world, when, in consequence of war or
commerce,  nations intermingled indiscriminately with each o-
ther,  the colour of the iris was invariably peculiar to the inhab-
itants of different climates.   In persons of fair hair and com-
plexion, it is generally of a grey or blue colour, and in those
who  have black or dark  hair, it generally  observes various
shades of hazle.  The choroid coat is,  externally, of a brown
colour,  and  internally, is covered by a fine black  pigment, ad-
hering slightly to its surface.  From the ciliary band)  and im-
mediately behind the uvea, the choroides  produces, transe-
versely and backwards,  an extremely  fine  membrane, to be

          *  HaUer on Phys. Chap. I6./1. 24,7.
          t  Ibid, Chap. 16. p. 248.
          \  Monroe on the Eye. 
(  9  )
connected, apparently, to the capsule of the chrystaHthe lens,
where, in order to be adapted to a smaller ck"cumference,i it
is drawn into numerous folds, of a fan-like figure, which are
called the ciliary processes.  After being connected t6 the lens,
these processes give out small points, of inconsiderable length,
which float loose in the posterior chamber of  the^qijeous hu-
mour, and have no immediate connexion with the lens.  The
ciliary pfioceSses are covered by the same black pigment which
lines every other part of the choroides.   The  third coat is the
retina.  This is formed by the expansion of the medullary part
©f the optic nerve, over the internal surface of the choroides,
and proceeding along the ciliary  processes, terminates  at "the
outer edge of  the chrystalline lens.* This coat is of the first
consequence, in vision ; and any defect in it, is of the greatest
injury to sight. Within all these coats,  are contained fine pel-
lucid fluids, termed by anatomists, humors.  They possess
different refractive densities, and are divided into three kinds,
viz. the aqueous, chrystalline, and vitreous humors.
   Before I speak particularly of  these fluids,  it will be neces-
sary to describe  the cornea.  This, we  have seen, is part of
the sclerotic coat ;  it is perfectly transparent, and consists^ of
several plates,  filled wtth an extremely  pellucid water. It is
the segment of a sphere, smaller than that which the ball of
the eye, in general, describes, and, of course, projects farther
forwards than  any other part of its surface.  The aqueous  hu-
mor is situated in the anterior and posterior chambers of  the
 eye, or all that space,  first, between the iris and cornea, and
secondly,  between  the uvea and chrystalline lens, communi-
cating at the pupil.   The chrystalline humor, or lens,  is of a
double convex figure, having its posterior  side  most convex,
and is inferior  to no known substance,  in transparency.  It is
composed of numerous lamelLx, which become much denser,
as  they  approach  its centre.  The whole is surrounded by a
very fine capsule, with a small  quantityof transparent fluid in-

     *  Monroe on the Eye, chap, 3. sec»<i. p.96. 
(  io   )
  tervening. The chrystalline lens is situated immediately behind
  the pupil, and is sustained behind, by the vitreous humor, and
  laterally, by the ciliary processes.  Its capsule is connected to
  that of the vitreous humor, on  which, as it  were,  by its pres-
  sure,  it forms a superficial cavity,  and lies  in  it very secure-
  ly.    The  vitreous  humor occupies all the  space between
 the chrystalline lens and retina, or what is usually termed the
 bottom of the eye.  This fluid is also contained in  a capsule
 called hydloides,  with  intermediate  cells,  formed  by a fine
 membrane, of which there are many, but they do  not observe
 any uniform figure. The use of these  cells, is probably,  to pre-
 serve so large a quantity of fluid from agitation.  This humor
 is also perfectly transparent, allowing the rays of light to pass
 through to the bottom of the eye, in their primitive purity.

   The optic nerves are two large nerves arising from the tha-
 Iami nervorum opticorum, and passing thro' foramina in  the
 spheroid bone, enter the ball of each  eye at  about twenty six
 degrees from the axis of vision, and sending off their medullary
 fibrils, on the internal surface of  the  choroid coat,  form the
 retina.

   The principal blood-vessel of the eye,  is the opthalmic arte-
 ry, a branch of the internal  carotid.   It enters with  the optic
 nerve, first giving off branches to supply the  sclerotic coat
 and  integuments,  and sends  off branches  to  the  external
 surface of  the choroides, which form  ramifications,  and pro-
 ceed forwards, to  supply  the circle of the uvea, and ciliary
 processes.   A small artery also  emerges  from the  opthalmic,
 or from some of its  branches, and penetrating through the me-
 dullary part of the nerve,  spreads over every  part of the  outer
 surface of the retina, forming a close net-work, so firmly inter-
woven, that anatomists have  sometimes mistaken it for a mem-
brane.   This is called arteria centralis.  The chrystalline lens
receives a branch from this  artery, which passes from the re-
tina, through the vitreous humor,  and perforates its posterior 
(  11   )
part.   The veins,  in general, run more externally, than the ar-
teries.  The pellucid vessels are very numerous,  and difficult
of demonstration.


                       SECTION II.
                       Of Light.

  AS vision depends entirely upon the mutual action of the
rays of light,  and refracting humors of the eye, upon each o-
ther ; and as there are certain immutable laws, by which such
actions  take place, it is highly necessary to consider the pro-
perties of light,  in general,  before we attempt to explain any
theory of vision.  This necessity only, could impel me to treat
of a subject whose difficulties require a much abler pen  ; and in
arranging my subject in this order,  I follow-the example of the
best writers on the eye, particularly Dr. Porterfield.

  Light is a substance, sui generis,  emitted  from luminous bo-
dies, in right  lines,  with  an amazing velocity.  The rays are
so infinitely small, that Philosophers  have  never been able  to
assign to  them, any correct  physical breadth;  and notwith-
standing this,  each ray is capable of being  divided into seven
smaller rays,  possessing different colours.  All transparent
bodies,  which allow the light to be freely transmitted through
them, are called mediums, and light is only affected when pass-
ing through mediums which differ in density.*   When a beam
of light passes from one medium, into another, it is denomi-
   * Although the refractive power of mediums, is governed by
their respective  densities, yet those  bodies which are composed of
inflammable particles,  act more strwgly on the rays of light, than
                                                     othci # 
(  12   )
 nated the incident beam,  and its termination on the surface of
 the second medium, the point of incidence.  Any angle which
 the incident beam makes, with a line drawn perpendicular to
 the point of incidence,  is called the angle of  incidence ; and
 the angle which it makes by reflection or refraction, with the
 Same perpendicular, the angle of  reflection or refraction. The
 angle of reflection is always equal to that of iaeidence, and the
 above angles are found  invariably in the same plane.  When a
 ray of light passes from a  rarer into a denser medium, it is
 bent or refracted towards  the perpendicular, or that line, before
 mentioned, drawn through the point of incidence ; so that the
 angle of  refraction will be less than the angle of incidence ;
 and if it passes again into a rarer medium, it will be refracted
 from the perpendicular, and the angle will be accordingly.

  To the above laws,  I  may add, that every object which is
 seen by reflection, or refraction, is always perceived  in the
 place from whence they were last reflected, or refracted.

  No philosopher  has been so successful in the investigation of
 the properties of light,  as Sir Isaac Newton ; and to his expe-
 riments, I shall pay particular attention, in the course of this
 Section.  In all his experiments to ascertain the refrangibility
 and reflexibility of the rays of light, he used oblong pieces  of
 glass, with three plane polished sides,  forming a triangle, call-
ed a prism.  He first proved that rays which differ in colour,
                                                      differ
 others of a milder nature, in proportion to their densities.  Sir I-
 saac Newton supposes, tliat as sulphur, placed in the focus of a
 burning glass, is very soon inflamed ; and as all action is recipro-
 cal, that sulphur ought to act, in a very high degree, upon light:
from whence he concludes, that bodies act upon light, in proportion
 to the sulphureous particles which they contain.
                            Newton's Optics, Page 248. 
(  13  )
differ also in refrangibility.  By viewing, through a prism, a
black piece of paste-board, divided by a line into two parts, one
part painted with a blue, the other with a red colour, and  this
paper being placed so that its sides were parallel to the prism,
and both parallel to the horizon,  and the cross line also, and
the angle which the incident light from the window made with
the paper was equal to the angle which the reflected light from
the paper made with the eye, the walls of the chamber round
the window having been previously made black, in order to pre-
vent any reflected light passing to the  edges of the paper* and
confusing the experiment.   Then turning  the  prism slowly
on its axis, he found that the blue half was lifted higher by the
refraction than the red half, and by turning the refracting an-
gle downwards, the blue half was carried lower than the red ;
from which he concluded,  that the two  colours  differ  in re-
frangibility, and that the red is less refrangible than the blue.

   He  next proceeded to prove that the sun's light consists of
rays differing in refrangibility, by allowing a single beam of the
sun's light to pass through  a prism,  to a plain surface station-
ed.to receive it, where it instantly exhibited an oblong spectrum
ofj colours,  of  which he counted seven, and these colours ap-
peared in the following order,  viz. red, orange, yellow, green,
blue,.indigo, violet.  Now the  angles  which these rays made.
with {he. perpendicular, were different, each making a separate
angle, consequently their degrees of refrangibility were various,
aad.ihe violet making  the greatest angle and the red the least,
thejformer is most and the latter  least refrangible ;  the other
rays possessing their degrees of refrangibility in intermediate
proportions.  These rays  were immutable, no  second refrac-
tion being capable of making any  alteration in them,  and the
above lie proved by a variety of  experiments.  But as the seven
homogeneal rays in the coloured spectrum, were mingled  so
much with each other, tliat they appeared t© the eye in a cer-
tain degree heterogeneal, and as this was caused by the circles
                             C 
(  14  )
 of colours answering to the diameter of the sun's disque, inter-
 mingling with each other ; he considered, that if the  diame-
 ters of these circles were lessened, their centres retaining their
 positions as before, that the mixture  would be proportionally
 diminished.  To accomplish which, he has  given the following
 experiment.  Into his dark chamber he admitted a beamof the
 sun's light, and at the distance of  ten or twelve feet, he pla-
 ced a convex lens, which cast the image of the  hole, upon a pa-
 per situated at the focal distance from the lens, and then pla-
 cing between the lens and paper, near to the  former, a prism,
 the light was refracted towards a second paper and there formed
 a spectrum.  This spectrum was, as usual, of  an oblong form,
 with rectilinear sides, but the  mixture of homogeneous rays
 was considerably lessened, and  the circular images of the hole
 were distinctly terminated, without any penumbra.   Thus, by
 lessening the magnitude of the sun's disque, apparently, by the
 hole in  the  window shutter, the circles in the spectrum no
 longer answered to his whole diameter, but only to that which
 appeared through the hole, and the lens,  by making that dia-
 meter less, rendered the  circles more distinct, as above.  In-
 stead of the round hole in the window shutter, he recommends
 one  in the shape of a long parallelogram, or of a  triangle
 whose two sides are equal, and eight or ten times longer than
 its base, which may be -^ part of an inch,  for if the  light be
 admitted in this manner the  homogeneous lights will be larger
 and more convenient for experiments.  The lens ought  to be
 good, and the prism should have an angle of 65 or 70 degrees
 at least,  free from veins and truly plane, and the chamber ought
 to be perfectly  dark.  In  performing the above experiment,
 although my chamber was perfectly dark, I did  not succeed
 so well as I wished, by reason of a great  deal of reflected light
 from the clouds, which entered the hole in my window shut-
ter, along with the  sun's beam.   I therefore made use of small
 tubes, blackened on the inside, which I fastened in the shutter,
and allowing the beam to pass  through, all the reflected  light 
(  15  )
was absorbed by the sides of the tube and the experiment suc-
ceeded much better.

  When objects are viewed through a prism or any other re-
fracting body,  they always appear confused;  which is owing
to the various refrangibility of the rays;  for  if flies, the letters
of a book, or any  other minute objects are  placed in any one
of the homogeneal lights, and seen through a prism, they will
appear perfectly distinct.

  The sine of incidence of every ray, considered separately, is
to its sine of refraction, in a given ratio.   This is sufficiently
evident from the  experiment I have  related;  for  there,  at
equal angles of incidence, each ray made a separate angle of
refraction; and Sir Isaac Newton further proves this, by form-
ing an hypothesis, assisted by a geometrical calculation, that
bodies refract light by acting on its rays, in lines perpendicular
to their surfaces.*  All homogeneal lights  are unchangeable
by  reflexion or refraction, and each homogeneal ray possesses
a capability of being reflected, in proportion to its refrangibi*
lity.  When a beam of light  is refracted by a prism, and the
spectrum of colours collected by a lens into a focus, a white
beam will be formed equal to the incident  beam.  Every white
beam is composed of seven rays, in different proportions, which
when divided by  lines into  seven spaces,  agree with the divi-
sions of a musical chord; and in all attempts to constitute a
white beam,  the above proportions must  be observed, or the
beam will be coloured with the superabundant light.

  The colours of all natural bodies are caused by their absorb-
ing some of the homogeneal lights, and reflecting others;  and
the surface of every transparent body reflects light, in propor-
tion as it differs from the medium with which it is in contact,
in refractive density.
* Newton's Optict, page 68. 
                        (   16  )
 - Having considered (although superficially) some of the most
obvious phenomena of light, it will now be necessary for me to
enter into an explanation of those causes by which they are
produced.  Light is not reflected or refracted by impinging on
the solid parts of bodies.  The truth of this may be made evi-
dent, by  a  great number of experiments:—-For example—
When a spectrum of colours is thrown upon a second  prism,
placed in such a manner as  to refract all  the rays,  and that
prism is turned gently on its axis, in  order that it may be ob-
liquely inclined to the incident rays, the most refrangible lights,
(such as the blue and violet,) will be totally reflected ;  while the
red, orange,  8cc. will continue  to be transmitted, and by in-
creasing the inclination of the refracting surface of the prism,
they will be all reflected, one by one,—the red last of all.   It is
not reasonable, therefore, to suppose, that at the same obliquity
of incidence, the red rays should find pores enough in the glass
for its transmission, and the violet and blue meet entirely with
solid particles. . When light is incident, out of air, on a plate
of glass, so obliquely as to be easily reflected, by placing water
on some parts of the lower surface of the glass—at those parts,
the light which  was previously reflected, will be transmitted ;—
therefore, the reflection was not caused by the solid  parts of
 the  glass, but by some certain constitution of the medium with
 which it was  in contact.   That this  constitution exists in all
 mediums, and also to a certain distance from their superficies,
 is demonstrable, by a property of light which/ I have not yet
 mentioned;  I mean that property, by which,  in passing  near
 any body, it is deflected or turned out of its direct course, which
 is generally called Inflection.—For example—When a beam
 of the sun's light, is admitted into a  dark chamber,  and the
 edge of a knife, a hair, or any other fine  substance, is placed so
 as to intercept part of the beam, the shadow of the intercepting
 body, is  found to be much broader, in proportion, when the sur-
 face upon which it is received, is situated contiguous to it, than
 when it is removed to a greater distance from it.  At the edges
 of these shadows, appear three fringes of colours, parallel to 
(  17  )
each other, which vary in their order, but are always so ar-
ranged, that the most refrangible rays are nearest  to  the sha-
dow.   If the blade of a knife is placed in  the beam, its plane
intersecting it at right angles, and part of the beam is  received
on the blade, while the  other part is  suffered  to pass by  the
edge of the knife, the light,  passing in  this mariner, will send
forth, at different angles, into the shadow, two faint luminous
streams resembling the tails of comets.

   The limits of this essay, will not permit me to describe the
variety of experiments  made on this  subject, by Newton,  Mi-
raldi and  others:  it is sufficient  to say, that they  all tend to
prove that bodies possess a power of  acting upon  the rays of
light,  by attracting them, at certain distances ; and at greater
distances repelling them; and in the experiment made with
the knife, the repulsive power existed at the distance  of about
the 800th part  of an inch ;* therefore, the two streams were
produced by one part of the beam passing within the sphere of
the attractive power, the other, of the  repellent.  The circum-
stance of the shadow differing so much, at various  distances
from the object, was easily explained ;  because, as the above
forces were  only  exerted at certain  distances from the  sub-
stances used in the experiments, all  those rays which passed
beyond their spheres of repulsion, would consequently proceed
in' their respective courses, unaffected by their action.  But it
was necessary  to account for the separation of the rays, as they
appeared in the fringes ;  and this was done by the hypothesis,
—that the least refrangible rays were acted  upon at greater
distances^ than those more refrangible; and consequently, the
violet and blue were nearer the shadow, than the others.

    All the above  circumstances tend to elucidate, in a most
admirable manner, the  cause of reflection and  refraction ;
which may be  thus explained—and first, we must admit,  that
* Newton's Optics, page 302. 
C  18  )
all bodies possess a power of acting upon the rays of  light,
similar to that which was shewn to be the cause of inflection :
—That these attractive and repellent forces are extended over
the surfaces  of all refracting and reflecting mediums ; the re-
pellent power existing at the greatest distance; that these powers
act in lines perpendicular to their surfaces ; and that all bodies
refract and reflect light, by the same power, variously exerted,
in various circumstances*  must also be admitted, because no
more causes ought to be acknowledged in philosophy than are
sufficient to  explain  the  nature of the phenomena.  Let us
then suppose, that a ray of light is passing through air, to the
surface of the denser medium, glass, at an angle of 40 degrees
with the perpendicular  at  the point of incidence; it will first
come into contact with the imaginary surface in which the re-
pulsive power exists, and this force acting in the perpendicular,
upon the oblique ray, will, according to the laws of motion,
cause it to describe a curve round the point of incidence ; and
when the incident ray, in describing this curve, has passed the
point of incidence,  it will  fly off, assisted  by  the repellent
power, at an  angle equal to that  of incidence.   This is  what
takes place in all reflections.   But when the ray  is incident
at a less degree of obliquity, its motion will of course be more in
the perpendicular, and  consequently will be better enabled to
resist the action of the repellent  force,  and will only have its
velocity slightly  diminished,  and its direction altered in pro-
portion to the resistance.   But  this velocity will  be immedi-
ately  increased when it arrives  within the sphere  of the  at-
tractive force, and it will enter the denser medium at an angle
differing from that of incidence,  in proportion to the power of
attraction and repulsion, which the body possesses, or in other
words, in proportion as  the superficies of the  refracting body
intercedes mediums which  differ most in refractive density.
Keeping the  above observations in consideration, we can very
readily conceive the  course which the  ray  takes  in passing
* Newton's Optics, page 244. 
(  19   )
from the denser medium, into the rarer, because it is then ex-
posed to the same actions as before ; but in this case it arrives
first at the attractive power, and will have its direction and ve-
locity changed, according to the resistance, but meeting again
with the repulsive  force, its velocity will be increased, and it
will fly off at an angle greater than that which the incident beam
in the denser medium,  made with the perpendicular ;  that is,
in  passing  from the denser into the rarer medium, it will  be
refracted from the perpendicular.   In prisms, the  obliquity of
the refracting surfaces to the incident light, causes the separa-
tion of its constituent parts, because, at certain angles  of obli-
quity, the seven primaiy rays are not equally able to resist the
attractive and repellent forces.  The rays of light are also se^
parable into their homogeneous parts, by becoming incident on
mediums of a certain degree of thickness, inclosed in others
which differ from them in refractive density.  For example,—
If a convex glass is laid on a plain piece of glass, they will only
come into contact at  a certain point, and to a certain distance
round that  point, will be interposed a  thin plate of air, of va-
rious degrees of thickness, according to the distance from the
centre or point of contact,  exhibiting circles of colours,  each
circle varying, in some measure, the  order of  its colours,  as
it is remote from, or contiguous to, the common centre of all
the circles ; therefore, light, incident on plates of this kind, is
disposed to  be reflected or transmitted,  according to the thick-
ness of the medium upon which it is incident, and these dispo*
sitions always return at certain intervals:—or, light is disposed
to be reflected,  as the thickness of the medium, is to the num-
bers, 1,  3,  5,  7, 9, 11,  &c and to  be  transmitted at all the
intermediate thicknesses, as, 0,  2,  4, 6, 8, 10, &c. for it was
observable,  that between the rings  of colours,  when the light
was transmitted, the circles of colours formed by the transmit-
ted light, were all those  which were wanting to compleat the
colours of the reflected light.  These effects will be produced if
the thin plate be denser than the surrounding medium, such as
a soap bubble. If one of these is covered by a clear glass globe. 
(  20  )
    to prevent its being agitated by the external air, the same  va-
    riety of colours will be seen, in  succession, as the water sub-
    sides from the top to  the bottom of the bubble, until, at length
    an intensely. black spot will appear at the top, because  at that
    place it becomes so thin, that all the light is transmitted, and
    immediately afterwards, the bubble breaks. Glass, also, blown
    very thin, exhibits  the same  colours much more  lively ;  for
    thin transparent  bodies, inclosed in a rare medium, will, ex-
    hibit more vivid colours, than those which are rarer.*'

      The least  parts of  all natural bodies, are transparent;  for
    gold,  altho' a very dense  substance, when  made extremely
    thin, will allow part of the  light to  be transmitted.  Between
t   the parts of opake bodies, a great number of spaces exist, con-
    taining mediums  differing  in  density from the  bodies  them-
    selves ; for it is by this order of things, that light is suffocated
    in opake bodies, by  the numerous  reflections made in their
    internal parts.

      From what has been  said  respecting the colours of thin
    plates,  it will be easily conceived, that a certain constitution of
    the parts of bodies,  and their intermediate spaces, with respect
    to size, must exist, to render them either opake or coloured ;
    and  from the same causes,  must reflect one species of colour,
    and transmit all the  rest; for one degree of thickness will inva-
    riably  reflect one kind of colour.  Upon these principles are
    founded the colours of all natural bodies, with which the  whole
    face of nature is enlivened.   The variegated plumage of the
    feathered tribe :—the beautifully vivid colours presented by the
    western horizon at the setting of the sun :—the various tints
    of gold and purple, which the clouds display at that period ; and
    the vast quantity of coloured light reflected in a clear evening,
    from the  snowy  summits of distant mountains, all originate
    from the reciprocal  action which the great Author of nature
    has ordained to exist between bodies and li<* ht.

                 * Newton's Optics, p. 195. 
(  21  )
  Rays of light emanating from luminous points, are naturally
divergent; but these rays may be rendered more divergent, or
made to converge, by refraction, so as to meet  in points simi-
lar in number and colour to those from which they proceeded;
for  this purpose,  pieces of glass,  whose surfaces are smooth
and regularly convex or concave, are used, called lenses.

  Lenses are of various kinds, according to  the purposes for
which they are designed.   Of those kinds it will be sufficient
here to consider two:—I mean the double convex and the con-
cavo-donvex.  The former is used to refract the  rays  issuing
from all points of an object,  and collect them into a focus, at
which place they  will represent an image of the object;  and
the distance of this focus, from the lens, is always  in  propor-
tion to its convexity.  If a convex  lens is fixed in the window
shutter of a dark  chamber, and apiece of paper or any other
white body be placed  in the focus of the lens, a compleat and
lively representation of the objects situated outside of the win-
dow, will appear on the white surface, in an  inverted position.
In this case, all the rays  emitted  from the  radiant points of
the external objects, which  are incident on  the surface of the
lens, are refracted,  and converge  so as to meet in the same
number of points on the paper ;  but as they necessarily cross
each other before they arrive at their respective foci,  they be-
come all inverted.  This is properly  a camera obscura ; and
upon  the same principle, all those are made, which are found
so useful in drawing perspective views, Sec.  If the rays are in-
cident on the convex surface of a concavo-convex lens, they will
issue  from  the concave surface extremely divergent, and this
divergency will be  in proportion  as  the two surfaces of the
lens, differ from each other in their degrees of curvity.
l>
Section 3. 
(  2S  )
                     SECTION III.

                         OF THE

   MANNER \IN WHICH VISION IS PERFORMED.


  There is a certain innate principle) inseparable from humani-
ty, which naturally leads us to contemplate the works o| nature
with a peculiar gratification.  There are few periods in our
lives, in which we experience a more serene satisfaction, ,than
when, in solitude, we are occupied in viewing the stupendous
height of  the distant mountain, or the foaming torrents of the
rushing cataract.  If we turn our attention t$ the  diversified
streaks of the, tulip, or the flinty crags of the rock, we are alike
intuitively impelled to admire and acknowledge*  the wisdom
by which they were constructed. The uncultivated peasant will
gaze in silent admiration, on the colours  and grotesque figures
of the evening clouds, with emotions which his simple, language
is incapable of describing : but it is only the outlines of nature
which can give satisfaction to minds  uninformed.  The man of
genius and philosopher  alone can become acquainted with her
diversity of tints and shades, and discover the secret touches
which are imperceptibly the foundation of all the beauties in
the picture.

  The subject of this section, and to which the two foregoing
ones were only preparatory, is highly calculated to excite more
interesting reflections, than any other phenomena with  which
we are acquainted.   Having in  the  first section described the
form and situation of the cornea and humors of the eye in gen-
eral, I will now proceed to speak of them in a more particular
manner.  And first, of the  aqueous humor.

    The aqueous humor is less dense  than any other refraoting
humor of the eye, and is situated in the  anterior and posterior
chambers, between the cornea and chrystalline tens. As the cor- 
(  23  )
nea is the segment of a concave sphere, having its concave sur-
face towards the aqueous humor, and the anterior surface of the
chrystalline lens, of a convex figure, it is plain, that the whole,
supported by the surrounding parts, must be shaped like a con-
cavo-convex lens. In consequence  of this shape, all the rays,
proceeding from an object, which become incident on  the cor-
nea, are refracted by the aqueous humor, and pass on thro' the
pupil, to the chrystalline lens. As the lens is of a double convex
figure, and a much denser medium, than the aqueous huthor^
the rays', in passing thro* it,  suffer a very great degree of re-
fraction.  Its posterior surface is  extremely convex, arid the
vitreous humor with which it is in contact, possesses a less de-
gree of refractive density ; inconsequence  of this, each  pencil
of rays, in proceeding thro' to the vitreous humor, are rendered
convergent, and meeting in a certain number of poihts on the
retina, imprint on its surface a correct inverted image of the
object.   The inversion of the image is owing tothe great de-
gree of convexity of the posterior part of the leris, which causes
the raysto;decussate each other about the centre of the vitreous1
humor. Thus  we may consider the humors as a compleat diop-
tric instrument, placed before the retina, and the whole  a per-
fect camera obscura.


                      SECTION IV.


        DISTINCT AND INDISTINCT VISION.   ;


   A distinct state of vision is (presupposing that the ejre is
in a perfectly healthy state) a capability of adjusting  itself to
remote and^contiguous objects, in  order that they may be im-
printed on the retina in a  proper focal image.   As some con*
troversy has existed* respecting the manner in  which this ad- 
(  24   )
justment is performed, it will perhaps not be unnecessary to
give some account of the opinions which have been entertained
on this subject.  M. de la Hire, supposed that the  eye suffered
no alteration, except in the contraction and dilatation of the pu-
pil ; and was supported in this opinion by  M. le Roi, who gave
as a proof, that by holding an object so close to the eye as to ap-
pear confused, and then placing a card with a small hole in it,
or artificial pupil, between  the eye and  the object, the latter
would then be plainly  discernable.   But  this hypothesis  was
found to be erroneous ;  for  although a narrow pupil  renders
vision less indistinct, it is only by^confiningthe bases of the pen-
cils of rays proceeding from the points of an object, and by that
means lessening their circles of dissipation on the retina.*  Dr.
Porterfield has advanced a number  of very ingenious  experi-
ments, by which he proves that we are capable of changing the
conformation of our eyes,  and of adapting them to various dis-
tances, and that this change  always follows a similar  motion
in the axes of vision. This change of conformation, he thinks,
consists in a motion of the chrystalline lens, by means of the
contraction of the ligamentum ciliare,  which increases its dis-
tance from the retina, according to the distance of objects ; and
as the ligamentum ciliare is convex, anteriorly,  by its con-
traction it loses,  gradually, all its convexity, and presses back
a portion of the vitreous humor, which, in  consequence of this,
presses against the  lens,  and assists in  moving it forwards ;
this last motion, of course, pushes the aqueous humor against
the cornea,  which is rendered more  convex ;  and thus we are
enabled to see near objects more distinctly.   To this it was ob->
jectcd, that the  ciliary ligaments are not muscular,  and  of
course,  have not the  powrer of contraction.  But  Dr.  Porter-
field defends his theory, by  saying,  that  his. opponents have
been led into a mistake,  by supposing that all muscles were of
a red colour.  This, he says, is not the case ; for the  muscu-
lar fibres of the stomach and intestines, have scarcely any  red-
ness in their colour.   He'continues,  it is also certain, that the
           * Priestley on  Optics,page 641. 
(  25  )
pupil contracts and dilates itself, according as objects are more
or less luminous,  and yet none  of the fibres which  perform
those actions, are in the least degree, red*   Dr. .Turin's hypo-
thesis, related by Dr.  Priestley, differs widely from the above.
He also supposes a contraction of the ciliary processes, which
he says, takes place when the eye is to be suited to greater dis-
tances than  15 or 16 inches ; and  in consequence of this con-
traction,  the anterior surface of the capsule of the chrystalline
lens, into which their fibres are inserted,  is drawn a little for-
wards and- outwards, which causes the water  within the cap-
sule to flow from under the middle, towards the elevated  part
of it, and  the  aqueous humor must, of course, flow  from the
elevated  parts of the capsule, towards the middle.  In conse-
quence of.this, the whole anterior surface within the  insertion
of the. ligamentum ciliare, will be reduced to a less convexity.
When this contraction ceases, the capsule will return to its
former situation by its own elasticity, and, being a very tender
membrane, containing water between its inner surface and the
lens, can readily obey the  effort of the ligamentum ciliare, al-
though so weak a muscle.  Dr. Priestley refutes this doctrine
by a very  just observation, that this alteration of the situation
of the water surrounding the lens, could make no change in
the direction of the  rays, unless it  was possessed of  a greater
refractive density, than the aqueous humor,* which is certain-
ly not probable.
   The experiments instituted by Mr. Evcrard Home, prove,
indisputably, that the eye  possesses a power of changing its con-
formation, independent of the chrystalline lens. By means of an
instrument invented by Dr. Young, which, by its  determining
with accuracy the changes which occur in vision,  is called an
Optometer, he ascertained that persons who have been depriv-
ed of their chrystalline  lenses can accommodate their eyes to
various distances ; md with the assistance of a convex lens, he
found that they possessed a state of vision  not differing mate-
rially from that to which they were accustomed when  their eyes
             * Priestley on Optics, pap;e 650. 
(  26  )
 were perfect.  The optometer is made by drawing, on a piece
 of paper or pasteboard, a black line, about three feet long, and
 placing  the  paper in a horizontal position, the person who is
 performing the experiment must look along the black line thro'
 two slits made in a card, so close to each other as to be within
 the limits of the pupil and perpendicular to the paper.  Behind
 the card, may be  placed, occasionally, a small convex lens, and
 holding it close to the eye, by viewing the line at a point situ>
 ated a few inches from the  farthest extremity, he will perceive
 the line divided into two, and appearing to cross each other at
 the place of observation. After marking this point of decussa-
 tion, he must change the conformation of his eye, by looking
 at the line, a few inches from its nearest extremity; and jf his eye
 is capable of altering its conformation, in proportion to the dis-
 tance, he will perceive the crossing of the fines, as before.

  Therefore, the power which the eye possesses, to adjust itself*
 to the distances of objects, may be exactly determined by this
 instrument. For the faculty with which  it is endowed,  in rJhi$
 respect, is always in proportion to the distance of the two point*
 of decussation from other.

   Mr. Home directed a person who had been deprived of .-the,
 chrystalline lens, to make the experiment with tfie optometer in
 the above manner, and two  places of decussation were distinct-
 ly observed by him. This man saw best in a strong light, aijd^
 his eyes  were much fatigued by viewing objects by candle light,.
 the reason of which is too obvious to require any comnjent in ;
 this place.*

 The observations of the ingenious Dr. A. Monroe seem to ren-
der all the theories  depending upon a motion of tiie  chrystat-
line^lens, in consequence of the contraction of, the ciliary pro-
cesses, highly improbable.   In his examinations Qf,th^retinar

    * Croonian Lett.  P/iil. Trans. R. S. 1802, part I. 
(  27   )
in order to determine its precise termination,  he discovered,
that instead of ending abruptly at the root of the  ciliary pro-
cesses, as had been supposed, it proceeded forwards on the in-
ternal surface of  the ciliary processes, and terminated at the
outer edge of the chrystalline lens,* and consequently, that the
ciliary processes do not form a compleat septum between the
aqueous and vitreous humors, and that the  chrystalline lens
has no siipport but what it receives from the  union of its cap-
 sule with that of the vitreous humor.
     -■>*■■■ i .■  .
   ft was necessary, therefore, to account  for the motions by
which we regulate our eyes to the distances of objects, in some
other manner.   This, he endeavours to prove, is accomplished
in a great measure by the  action of the  oblique  muscles and
orbicularis palpebrarum.  That  when the  oblique  muscles
contract, they make a strong pressure upon the ball of the eye,
and elongate its axis so as considerably to increase  the dis-
tance between  the  chrystalline  lens  and  retina.   He has
given some experiments to  prove, that when  we attempt to
discern very  contiguous objects, the orbicularis muscle con-
tracts, and by pressing upon the upper ahd lower parts  of the
cornea,  renders it more convex,  and consequently, the object
more distinct.   He placed a book so close to  his eyes that the
letters became indistinct;  and keeping his eyelids at  the dis-
tance of nearly half an inch apart, with his fingers, he was un-
able by any exertion of his eyes, to perceive the letters distinct-
ly!; he tried the same experiment by acting  only with the at-
tollens  palpebram superiorem,. but with the same effect.  He
then, at the same  distance  from the book, made an exertion to
read, by acting with the orbicularis palpebrarum, so as to bring
the Jedges of the eyelids within a quarter of an inch of each
other,"''arid discovered that he could see the letters and words
very plainly.  In  another  experiment, at the  same  distance
from the book, when the letters became indistinct, he brought
* Monroe on the Eye, chap. 3,/?. 96. 
                             (   28   )

 thenedges of his eyelids within a quarter of an  inch of each 0-
 ther, and then stretching them with his fingers so as to make
 pressure upon the upper and  lower edges of  the cornea, the
 letters appeared perfectly distinct.*

    I am disposed,  however,  to think that the,action of the o-
 blique muscles,  in assisting vision, by  passing ,upon, the* ball
                                               -    ■       " '■■-'"

   *  Dr. Hosack's observations on'vision, do not differ, essentially, from
 the opinions delivered by  Dr. Monroe.  He also advocates the action
 of the external muscles  in  producing a change in vision; but sornfe of
 hte inferences are perhaps not perfectly correct.   In  p4g& 18, of his
 paper, he  asserts, that in consequence of the elongation of the eye, pro-
 duced by the external muscles, " the media, viz- Thetaqusous, .chry^al-
 - line, and vitreous humors, through, which the rays pass, are also, length-
 ened ;.; of  consequence,  their powers  of refraction are. proportionally
 increased, all which correspond to the general principle."  But this
 opinion does not correspond with the general principles of optics: —
 For, as no change can take place in the density of the humors, by the
 elongation oftheir axes, no alteration can  be produced  in their re>firac-
 tive powers.  If the Doctor alluded to any change in  the conformation
 of their surfaces, this is equally inadmissible, (with  the  exception of
 the anterior surface of the aqueous humour.) It is very evident that no
 alteration  pccjirs in the ponvexity of the lens, inconsequence of the ex-
 ternal pressure ; and  unless this was the case, the surfaces, of the, media
 with which it is in contact, would remain unaffected, and consequently
 the light would be refracted as before.   For every change'which  takes
 place in the direction of the rays,  is invariably effected  at the surfaces
 of  the refracting bodies.  He also attempts to  prove that in persons
 who; squint, the inability of the distorted eye, to. perceive objects dis-
 ' tinctly at  the same distance at which they are visibjest©. the other, de-
 fends, upon  the  irregular contraction of  the  abductor and, ialdd,u.ctor
, inuscles..  But this hypothesis is rather inconclusive,, and by no means
 agrees with  the experiments I  have adduced in this essay.  If those
 persons who honour this dissertation with a perusal,  think that any
 other arguments are  necessary to  refute this opinion of Dr. Hosack,
 I nn st heg  leave to refer them to Dr. Smith's optics, and'M. BulFon's
 remarks upon this Subject,  related by Priestley-       ; 
(  29   )
of the eye, is rather improbable; but  even were it admitted,
their action, in this way, must be very limited, from the follow-
ing considerations :

  When the pencils of rays, refracted by the humors, arrive at
a focus behind the retina, or converge too much, so as to decus-
sate before they become incident on its surface ;—(either of
which takes place in all cases of indistinct vision not depending
upon amaurosis or opacity) in all attempts to remedy  this defect,
the axisof the chrystalline lens must move in the  direction of a
line drawn through the axis of vision, and must always observe a
position parallel to the pupil and cornea : for if  by any force,
it is moved sideways, out of the axis, or line drawn thro' the
centre of the pupil, or rendered obliquely situated with  respect
to the pupil and cornea, the rays, in either  case,  would, by the
unequal refraction, fall  in a very confused manner on the reti-
na. In order therefore, to refract the rays regularly, the chrys-
talline lens, pupil and cornea must be parallel to each other,
and each of their motions to regulate the focal image, must be
uniform.

   If these motions are produced by pressure upon the eye ball^
that pressure must be of an equal degree of force, on all sides ;
and to effect this, the situation of the  oblique muscles is very
well adapted ; but they can only make  an equal degree of pres-
sure, when they are in equal states of contraction, and this is
seldom or never the case.   If the eye is placed  obliquely and
consequently but one of the oblique muscles in a state of con-
traction, and that by this single contraction  it pressed upon the
eyeball, the vitreous humor would be pressed obliquely against
the Jens  and ciliary processes, and very  much discompose
their situation, which certainly does not occur.  The  same ar-
gument may be  advanced  against  any pressure made by the
recti muscles} unless they both act, uniformly,  together; and
when they act in this manner, the pupil is exactly in the centre
                             E 
(  3b   )
 of the eyelids*  The orbicularis muscle, Dr. Munroe says, as -
 sisfs vision by pressing upon the upper and lower edges of the
 cornea, and thus increases its convexity; but if we contemplate
 the cornea in this situation, we will find its figure very badly
 calculated to render vision  distinct.  Its form, in this case, will
 be elliptical,  resembling the form of an egg divided longitudi-
 nally; thus its horizontal convexity is lessened and its perpen-
 dicular convexity increased.  The consequence  of this kind of
 figure certainly would be,  that all  perpendicular objects unless
 very close to the eye, would appear much smaller than natural,
 and all those placed horizontally with respect to the eye, would
 appear confused :—a round object would seem of an oval form,
 and the rays proceeding from all bodies would be so unequally
 refracted, that no compleat image  could be formed on the reti-
 na. To prove  the correctness of some of the above  observations ;
 I made the following experiment.  In a tube, half an inch in dia*-
 meter, and twenty inches long, I  placed  transversely, several >
 pins, at various distances from each other, then fixing my head
 in  one position, and placing my eye very obliquely, I looked
 through the tube at the most distant pin, and gradually chang-  '
 ing the conformation of my eye, by viewing  each of the pins  :
 in succession, until I came to the last, which was distant near-
 ly four inches  from my eye,  I  found that I could perceive it •.,
 very distinctly without much exertion, and by fixing my eye in
 a horizontal direction very  much sideways, and looking at the
 pins through the tube, I always discovered the same result.

  Sitting  opposite to a window, I  fixed the tube before, me,
 firmly, in a horizontal position, and looking through it, so as
 that the pupil was directly in the centre of the aperture .formed
 by the eyelids, I then with my fingers, removed the upper and
 under eyelids, as far from the ball  of my eye as possible, and
 half an inch from each other, and making an exertion, to view
the pins in succession as before, I found that I could in this
manner discern the nearest pin  as distinctly as whe»  the orbi-
cularis muscle was £t liberty to act. 
(  31  )
   I next fixed a book so close to my eyes, that the letters be-
 came  indistinct; then moving my eyelids to the edge of the
 cornea, I stretched them with my fingers so as to make pres-
 sure upon its upper and lower edges in the manner described
 by Dr. Monroe, and the letters immediately became more dis-
 tinct but appeared to me much longer, in the direction of the
 lines, than natural.  The pressure which it was necessary to
 make ©n the cornea in order to produce this effect, was rather
 painful,  arid we do not find in  any ordinary  exertions of our
 eyes, the least degree of painful sensation.

   From the above  experiments, therefore, I am inclined to
 doubt  the action  of the  external  muscles in increasing our
 sphere of vision; and am disposed to believe that those powers
 by which the eye  is regulated  to distances, all exist within the
 eye  itself.  That motions occur somewhere  in the eye is evi-
 dent.  The causes of those motions I am incapable of demon-
 stratingj but will beg leave to offer a suggestion of a means by
 which it is very probable material changes may be effected in
 the.eye.  As the retina intervenes between the ciliary processes
 and chrystalline lens, the latter cannot possibly  be affected by
 the contraction  of the former, without injuring the retina;
 consequently It cannot be by a motion of the lens, that the eye
 is adjusted to the distance of objects.   Any changes, then,
 which  take place, must be in the convexity of the cornea, and
 to effect  this, there is nothing that I know, so well situated as
 the ciliary processes.  The probability  of their possessing a
 power of muscular contraction has been generally admitted,
 and their conformation has led me to think that  their fibres are
 arranged round  the  lens in repeated concentric circles, from
the ciliary band to their connection With  the retina near the
chrystalline lens.  The numerous folds into which this mem-
brane is drawn, seem to be intended to allow the fibres to con-
tract with more ease* for these are probably only present, when
the fibres are in a quiescent state, and disappear when they are
all in a state of contraction. When each of these fibres contract, 
                          (   32  )

 its diameter is lessened in proportion to the contraction,  asid
 the circle of the ciliary band to which they are connected must
 also be lessened, and consequently the cornea rendered more
 convex. In this order of things, the change of convexity of the
 cornea is regularly conducted, as it ought to be, to refract the
 rays  incident upon it, uniformly.  For the cornea is perfectly
 analogous to the object glass of a telescope, and those glasses
 must always possess a regular degree of convexity, in order m
 transmit the rays to the other glasses of the telescope, uniform-
 ly refracted.   Those who are  of  opinion that  it would be  im-
 possible for so tough a membrane as the sclerotica, to be af-
 fected by the  ciliary processes,  must recollect,  that before it
 arrives at the circumference of the cornea, it becomes much
 more thin  and flexible,  than  at any other part.  That  this
 mode of suiting the eye to different distances, is the same in all
 animals, is very probable ; for although in the larger and stron-
 er species of  quadrupeds, we  may find  the sclerotica  ex-
 tremely hard, yet it is very reasonable to conclude, that nature
 tias endowed their ciliary processes with a muscular irritability
 suitable to the rest of their system.

    The precise limits of distinct vision has been made a subject
 of very great enquiry t Dr. Jurin, particularly, was very suCcess-
* ful in his explanations.  He places the smallest distance of per-
 fect  vision, at 4, 5, or 6 inches, in the generality of eyes;  that
 ■jsV the distance'at which a pencil of rays will converge jp a phy-
 (sical point on  the retina;  and  the  greatest distance, he  deter-
 termined at 14 feet  5 inches.  This last opinion was the result
 of a calculation depending upon the size of the eye, the refrac-
 tive  property  of  its  humors, and the apparent interval of two
 stars, whose distance is known.   Dr. Porterfield ascertains the
 greatest distance of correct  vision  .to be only, 27 inches, ac-
  cording to his own eye. These very different opinions, led Dr.
  Priestley to make the following remark; " Several philosophers
  speak of the farthest as well as the nearest limits of distinct vi-
  rion—but this language is evidently improper; since, if an ob-. 
                          (33)

 ject be large enough to subtend  a sufficient angle at the eye,
 it .matters not at what distance it be placed, ?febejng- seen with
 equal distinctness. If persons be not short sighted, they can see
 by parallel  rays, and some even with converging ones.   What
 they mean  by the farthest 'limits'of distinct  vision, seems ,tp_ be
 the greatest distance at which a bbols, of a middle ^e4 print,
 jnay be read.  This is plainly Dr. Juiin's meaning, for he says
 that this limit varies with the size of the print."*  But  limits
 4p4jbtless may be fixed to the distance of objects, under certain
 circumstances, and from all the experiments related  by Doctor
 Priestley, and others, oh this subject, I would draw the follow-
 ing conclusions.  That all eyes have the power of adjusting
,■ themselves so as  to refract pencils of rays-incident on their cor-
 neas, $q a greater or less degree.   That some eyes will conse-
 quently be able to view an object distinctly, when placed at all
 the intermediate  distances between 4 and 37 inches (which is
. the distance that  a middle sized print is distinctly visible to my
„ own eyes) and other eyes will not perceive  the same object at
 more than 20 inches, or at a less distance-than 7 inches! rThat
 beyond those limits, objects are only distinct, in proportion to,
 their subtending  angles, and the force with which their rays act
 on the sensible retina, which, for the most part, depends on the
 quantity of light reflected from them, and their  colour.   That
 in our judgment of the apparent  place of objects, we are always
 liable toerror, and that experience:alone can rectify theseerrors.

    Dr/Porterfield alledges that the surest way of judging of the
 distances of objects, is, by observing the angles rnade with the
 optic axes ; for  he compares  our  eyes to, two > stations from
 ,whic1i distances  are taken,  and this he says is the reason that
 persons blind  of one eye* so often fail in  snuffing a  candle,
 pouring liquor into a glass, 8cc  ,' $ut in viewing very remote
 objects, this rule will by no means answer ;  forthen^the optic
 angles.are" comparatively too small for the mind t^io'Tm any

          py* Pncstltij on Optics, vote 2, page  650. 
(  34   )
judgment by them ;  and when this is the case, the only me-
thod, I believe, by Which we can form any  judgment of their
distance, is in proportion to the confusion of their outlines, and
faintness of their  colour.  This  rule has long since been ob-
vious to landscape painters.

  As every part of the human system is subject to various Stages
of imperfection and decay, in passing  through the different
stages of earthly existence, this decay is evident no where more
than in the operations of sight. The eyes of children are much
smaller than those of adults.  Their humors are of but little re-
fractive density, and their corneas are much more flexible than
at a later period.   They are  consequently enabled to see at
small distances. On the contrary, elderly people see objects at
a moderate distance,  better than  those  more contiguous,  and
this for the following reasons;—The irritability of  their mus-
cular fibres is much lessened ; every  part of their system be-
comes more condensed; and the fluids bear a lessproportion to
the solids than formerly.  The sclerotica, of course, acquires a
greater rigidity.  The aqueous humor, and fluids contained in
the pellucid plates of the cornea are in less quantity,  and the
cornea, deprived of its usual  support,  becomes flatter and  less
fit to refract the rays.   The chrystalline lens, in some measure
loses its convexity ; a slight degree of opacity also takes place
in the humors, and most probably the  retina itself,  loses, in a
certain degree, its former sensibility. Although all these things
invariably occur in old age, yet they appear sooner or later, ac-
cording to the^situation with respect to health and temperament
of different individuals.

  Some persons have their  corneas naturally  with too little
convexity, and are obliged to remedy that deficiency by convex
glasses, which are also used when the chrystalline lens hap-
pens to be too fiat.  On the other band, the lens and  cornea
sometimes possess a  degeee of convexity disproportionate to.
their distance from the retina ; and this defect is remedied by 
(  35  )
concave glasses.  Eyes of this latter kind are preferable to the
former; for as old age advances, they become less plump, and
at a time when persons of the former class are obliged to have
recourse to glasses  more convex, the latter are enabled  to lay
aside their concave ones entirely.  The influence of custom,
as in other things, is very evident in vision.  If a person is ac-
customed, for any considerable  length of time, to look at near
objects, his eyes will become less sensible to the impression of
remote objects, or rather, he will lose the faculty of adjusting
his eyes so as to have a proper impression of them on the re-
tina.  For the same reason, persons who are accustomed to
view remote  objects, see  better at great distances than other
people, as is  generally the case with seamen, travellers, &c.
and engravers, watchmakers, &c perceive contiguous objects
better than those at great distances.


                      SECTION V.
OF  THE  SEAT OF VISION.
  I have hitherto spoken of  the retina, as being that part of
the eye which receives the impression of the object in order to
transmit it to the sensorium: and  this opinion prevailed uni-
versally for a great length of time, until a very peculiar disco-
very of a French writer, led him to embrace a new hypothesis.
M. Marriote perceived that there was one particular spot in the
retina, which-was  totally insensible  to the impression  of the
rays of light.  And upon further examination  found, that this
insensibility existed only  at the place where  the optic nerve
enters the eye-ball.   He  placed upon a dark wall, a sin all
round  piece of paper, about the hekhih of his eye,and another 
(  36  J
 piece of the same size, about two feet on the right hand,  ra-
 therjower; then shutting  his  left eye, he receded gradually,
 keeping the other eye  fixed on the first paper, and when about
 the distance of 10 or 12 feet from the papers, he instantly lost
 sight of the second paper;  nor was this in consequence of the
 obliquity of its position  with  respect to his eye, for he could
 distinguish objects plainly, situated more obliquely.  This ex-
 periment was improved by M. Peoquet and M. Le Cat; but the
 most approved manner  of performing it at present is as fol-
 lows :  Place three small pieces of paper, as nearly in a horizon-
 tal line as possible, about  two  feet apart.  Then covering one
 eye, recede gradually from the papers in an oblique direction,
 keeping the other eye fixed on the outside paper next the cover-
 ed eye ; and when arrived at a  distance about five times as great
 as the distance of the  papers  from  each other,  the middle
 one will entirely disappear, because in that situation, the rays
 fall exactly on the entrance of the optic nerve, and the other
 two papers will remain  visible.  This discovery led M. Marriote
 to suspect, that  the  defect was  owing to the absence  of the
 choroides at the spot wherej-'be optic nerve enters,  and conse-
 quently that vision depended more (if not entirely) on the cho-
 roides, than the retina :—Another  observation confirmed him
 in this opinion, which was, that the retina is a transparent sub-
 stance, and of course transmitted the rays ; from which he con-
 cluded, that it was impossible for any substance  to be the pro-
 per seat of vision, which did not stop the rays in their progress;
 and he thought that the choroides was best adapted to receive a
 strong impression of the light. In this opinion he was support-
 ed  by M. le Cat, and afterwards by Mr. Michell, the latter of
 whom argued, that it is necessary, in order that vision be dis-
 tinct, that the pencils of rays flowing from  the several points
 of an object, should meet  in a focus corresponding in the same
 number of points,  which could only  take  place  on some uni-
 form surface, and that the retina being uniformly  nervous, and
almost transparent, does not present a proper surface; and that
its thickness would prevent any correct image being formed, 
C  37  )
 because if the rays came to a focus on its surface, they would
 again diverge, before they passed  through, and  eXcite mnch
 confusion.  A variety of other objections to the retina are re-
 lated by Dr. Priestley, who seems to coincide in the opinion of
 Mr. Michell.   But 1 really think that the latter, in attempting
 to advocate the choroides, has indirectly supplied us with many
 arguments in favour of the retina.  Particularly the following ~.
 —In the first place, he contends, that if the image is made by
 direct rays  on the nearest surface of the retina, a great degree
 of confusion must arise from the light reflected from the cho-
 roides, in  animals  in which it is white  ©r  coloured; and it
 would be impossible for vision to take place at  all, if the retina
 receives  the  impression from the  choroides,  in animals  in
 which it  is quite black ; and yet he says all those animals see
 more distinctly than others, in whom it is of a  lighter colour;
 and vision  is most probably governed by the same laws in all
 animals.   A  second argument is,  that no membrane in the
 system, is better suited to  receive  a compleat impression  of
 external objects, than the  choroides ;  and on  the other hand,
 the retina receives a very faint impression or none at all. For
 he supposes that light is not in the least acted upon  by the re-
 tina,  as its density  cannot be greater than that of the vitreous
 humor.   He then goes on  to advance, as very favourable to
 bis theory, the great diversity of colour of the choroides, in dif-
 ferent animals.  For example,—-Those terrestrial animals in
 which it is - necessary to see by night, possess  it of a white
 colour or nearly so:—Birds of prey,  such as eagles, hawks, Scc~
^requiring a very acute state of vision, have it of a  very
black colour; and in a great variety  of quadrupeds, it is of va-
rious  shades of green or blue ;—Lastly, in man, who requires
an accuracy in vision, at a medium between the rest of animat-
ed nature, the choroides is not so black as that of the eagle, nor
so-white as that .of.  the cat*  And   Mr.  Michell  also asserts
tha* the choroides is abundantly supplied with nerves, sufficient
to jxansmitany impression  made on it, to the sensorium.  But
 .  -  .                        Ft            "    ' 
                        (  36   )
Mr. Michell's opinion of the thickness and transparency of the
retina  was certainly  erroneous.   According  to Dr. Monroe,
the optic nerves, after entering the eyeballs to form the  retina,
change from a white to a cineritious colour.*   The distance of
the retina from the choroides is extremely small, and if we con-
sider  the great number of vessels every where interspersed
over its'outer surface, supported by  a membrane analogous
to the pia mater, we will find that a very small part of that dis-
tance is occupied by the retina; from which I must conclude
that a pencil  of rays meeting in the smallest physical point on
the retina, cannot  affect it materially,  when incident on its
surface, although it may,  as Mr. Michell supposes, diverge
from that point before it meets with the choroides.

   With respect to the slight  transparency  of the  retina not
favouring the impression of the image, I  must only  observe,
that it is only necessary for the finest ray of light possible,  to
be incident on the surface of so delicate and sensible a mem-
brane  as the retina, to excite the motion of the nervous influ-
ence.   That its thickness is so small, it is of no consequence
whether or not the rays incident on its surface diverge or  con-
verge, before they meet with the choroides, because they are
then beyond  every instrument of sensation by which the im-
pression of such divergency or convergency could  be  carried
to the sensorium, and there produce a confused idea of the cor-
rect image present on the retina; and, that as in all animals,
in which a distinct vision is necessary, the choroides is black,
so in man we find it nearly so, and this in order that any  acci-
dental rays which  fall upon it, may not be reflected back on the
retina,  and  render confused the correct image already repre-
sented on its surface.  But in animals who possess  a very  light
coloured  choroides,  it  is probable that a different  order of
things takes  place.  All those kinds of animals are impelled by
fiature to  seek their food by night, or in places where very  little
* Monroe on the Eye, chap. 3, p. 93. 
(  39   )
light is admitted, and all the objects of their prey are most ge-
nerally of a dark colour.  Every part of their choroides reflect-
ing back to the retina, all the light incident on its surface, must
there excite an uniform sensation to which I believe it is always
subject when exposed to the light: for which  reason they  are
very careful to avoid luminous objects, which always in them,
produces  painful  sensation.  As I have  observed repeatedly
in cats, when I exposed their eyes to a white  wall illuminated
by the sun, that their pupils instantly became  perfectly closed.
When a cat, for instance, has discovered, in  the night, a bird
or a  mouse, these bodies reflecting  very little  or no light,  a
proportional quantity of  her choroides   is darkened,  and  of
course a certain portion of the retina is devoid of the sensation
of light, which I believe  would allow the sensorium to judge
of the presence of the object as well as if the contrary  state of
things was to take place.   In those of the quadruped  tribe,
whose choroides are green,  they are found naturally to subsist
on herbage, and although  it may  destroy  a perfect accuracy of
vision, with respect to all colours except green, yet it certainly
assists them in seeking a species of food  which is so uniformly
of a green colour.  Indeed  we may derive a very strong argu-
ment in favour of the retina, by considering the great analogy
between  it  and the  nerve  of hearing,   which is also spread
through the vestibulum and cochlea, in a fine medullary tunic :
add to this that in the disease of amaurosis, and in diseases in
which, in consequence of inflammation or any other cause, the
optic nerves are compressed, vision is compleatly destroyed.
The above opinions in favour of the retina are nearly those of
M. de la Hire, from all which I would infer :—That the  cho-
roides acts a very  important part in vision.  That its colour is
admirably suited to the situation and habits  of each tribe of
the animal creation, and that the retina  alone is  tbe medium
through which they can have any idea of the presence of ob-
jects.
   Various opinions have been given by authors respecting the
manner in  which we judge  of the correct position of objects 
(  40  )
 when they are inverted on the retina.  Some referring it to a
 principle  of instinct, by which the mind judges that each pen-
 cil of rays proceeded from the opposite side,—others, that we
 trace the light in perpendicular lines from the place of the im-
 age on the retina.   But the circumstance of the medullary fi-
 bres of the nerves decussating each other at the  corpus  annu-
 lare, and beginning of the medulla spinalis, has led me to believe
 that a similar decussation takes place of the medullary fibres of
 the retina, at the entrance of the optic nerves into the eye balls,
 and if this be the case, the inverted impressions of objects on
 the retina would be transmitted to the sensorium, in the contra-
 ry, and consequently their correct position.   If  each fibre of
 the retina is so small as the 36-400 part of a hair as Dr. Mon-
 roe* mentions, it will be impossible ever to ascertain this point
 of anatomy, by any  microscopical researches.

         * Monroe on Osteol. page 328.




   To the  Medical Professors of this University, I must now
 beg leave, to address myself, by assuring them,  that the infor-
mation I have derived from the useful and important instruc-
 tion  which their lectures, at all times, afforded, will  be es-
 teemed and cherished by me as my most valuable acquisition :
 and that their individual politeness and attention  to me, during
 my stay in Philadelphia, will be always held in the  most  grate-
 ful remembrance. 
 
 
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