OB 
HEAT, 
Considered as the Retinal Intermediate of Light and Color 
Sensation. 
BY L. WEBSTER FOX, M.D., and GEO. %L. GOULD, 
Reprint from the American Journal of Ophthalmology,' July, 1886 ON HEAT CONSIDERED AS THE RETINAL INTERME- 
DIATE OF LIGHT AND COLOR SENSATION. 
Ii. WEBSTER FOX, M.D., AND GEO. M. GOTJLD, A. B. 
Any one who brings to the study of Light and Color Percep- 
tion any considerable training in physics and physiology, with 
an unbiased desire to explain the mystery that enshrouds these 
processes, must be astonished at the character and futility of the 
theories advanced to explain them. Let such an one read a score 
or more of the multitude of expositions and criticisms relating 
to the two theories that seem to occupy the greatest share of 
public attention, and if he has, apart from the mere facts inci- 
dentally learned, gained a single inch of advance into the com- 
prehension of the manner and law of the transmutation of ethe- 
real vibrations into color, we heartily congratulate him. In the 
language of an elder pious investigator, 4 ‘The connection betwixt 
our ideas and the motions excited in the retina, optic nerve and 
sensorium, is unknown to us, and seems to depend entirely on 
the will of God.” . 2 
L. W. JFox—G. M. Gould. 
It is not our purpose to enumerate the criticisms made, or 
possible to be made, of the Young-Helmholtz and of the Hering 
theories. That has been done iterum atque iterum. Of the first 
mentioned, Wundt pithily says: “Genauer betrachtet sagt jene 
Hypothese gar nichts Anderes als was schon im Mischungs- 
gesetze enthalten ist, aber eine Erklserung des letzteren enthselt 
sie nicht; denn warum aus den drei Grundfarben alle Lichtem- 
pfindungen zusammengesetzt werden koennen,dies wird durch die 
drei Fasergattungen ebenso wenig begreiflich gemacht wie 
durch das Newton’scheDreieck.” 1 
A similar judgment is pronounced upon the Hering theory 
which, though to be preferred of the two, really succeeds in 
nothing more than in restating the problem in other terms. 
These theories, although devised by practical color students, 
and in the full light of the nineteenth century methods of induc- 
tive science, seem to be entirely of the nature of deductions, su- 
perposed upon the facts, regardless whether they fit or not— 
much like the Hegalian metaphysic. Their vagueness excites 
suspicion at once. One is often in doubt whether the location 
of the process is situated in the retina, in the optic nerve or in 
the brain; the Sehsubstanz is said to extend from the retina to the 
brain, inclusive of both ends. The theory is made large enough 
to cover all possibilities of future discovery. The Roman’s ox- 
hide can be made to cover many acres, if slit into a large enough 
network. Anatomical and physiological differentiations of func- 
tion are ignored, and we wander in a maze of glittering gener- 
alities which seem oblivious of the elementary principles of 
physics and physiology. 
But it is easy to be seen that the carefullest and largest minds 
are beginning to react against such a priori methods of proced- 
ure, and it is comforting to find that Wundt, the greatest liv- 
ing physiological psychologist, sets these theories aside and con- 
tents himself with the somewhat indefinite outlines of a theory 
1. This hypothesis when scrutinized more closely contains nothing more 
than the law of the mixture of colors—no explanation of this law. It as 
little makes comprehensible why out of the three primary colors, all color- 
sensations can be produced through the mediation of the three kinds of 
nerve-fibres, as does the deltoid color-chart of Newton.” On Heat. 
3 
that is not a mere figment of the imagination, but which in a 
large sense tallies with and is justified by the facts of ethereal 
vibration and physiological processes analogous to related phe- 
nomena in other functions of the body. The necessary indefi- 
niteness of the subjoined epitome arises from the healthy en- 
deavor to keep the theory within the bounds of fact, and if it 
does little more than state in succinct form the laws of these 
facts, such indefiniteness does not posit other and unknown mys- 
teries for the known ones till confusion becomes infinitely worse 
confounded. Wundt is too trained a sportsman in the open 
fields of physics and physiology to adopt any shot-gun theory 
which, fired at the facts broadcast, might, by simple dispersion, 
strike some game however wide of the mark. 
For the benefit of those readers who have not Wundt at hand 
we add a translation from the last edition in which he summa- 
rizes the elementary principles of his theory. 
1. “Every retinal stimulation results in two different pro- 
cesses, a chromatic and an achromatic; the first is a consequent 
of the wave-length of light, and the second is likewise depen- 
dent on the same, in so far as relates to its relative strength, the 
maximum of its intensity being reached in the yellow space of 
the spectrum. Both movements with increased stimulus follow 
different laws, the achromatic process beginning with a weaker 
stimulus, but soon surpassing the chromatic in intensity. In the 
middle degrees of light stimulation the relative strength of the 
chromatic process increases, but falls behind the achromatic 
again under the more intense degrees. 
2. The achromatic movement consists in an uniform photo- 
chemical process, whose intensity is in part dependent upon the 
above mentioned degree of the objective light stimulus, and in 
part upon the wave-length, reaching its maximum in yellow, and 
sinking toward both ends of the spectrum. 
3. The chromatic movement consists in a multiform photo- 
chemical process, continuously changing with the wave-length, 
and therein presenting itself as an approximate periodical func- 
tion of the wave-length, since the extreme differences of these 
produce approximately similar effects, while the effects of cer 
tain intermediately placed differences are opposed in such a way 4 
L. W. Fox—G. M. Gould. 
that complete compensation is brought about as opposed phases 
of one and the same movement. 
4. “Every photochemical process survives the stimulus for a 
time, and exhausts the sensibility of the sensory substance for 
that certain stimulus. The positive and similarly colored after- 
images are explained as the immediate results of this continu- 
ance of activity, whilst the negative and complementary after- 
images are explained by the consequent exhaustion.” 
So far Wundt, who plainly implies that the present state of 
inductive science does not justify any more decided specializa- 
tion and detail as to the “photochemical process.” In that word 
lies the crux of the question, and to solve the mystery hidden in 
it must remain the endeavor of every investigator. Our own 
aim in this paper is to ask if the term be not ill-chosen, and to 
propose another view, one, as we think, more in harmony with 
physics and more analogous to other physiological processes. 
It will be seen that we accept and stand upon the whole of the 
rest of Wundt’s theory, since it but formulates in a general way 
the known facts of the results of the impact of ether-waves upon 
the retina. We only ask in substance if a more definite and ex- 
act and known term cannot be given for the unknown and in- 
definite one derived from photography. 
The objection to the term, photochemical, is that, in the first 
place, there is little or no evidence that the result of ether-wave 
impact upon the retina is in any sense chemical. It implies a 
decomposition and a recomposition of retinal substance, which 
seems too gross a conception in view of the delicate structure 
and functions of the retina; and would necessitate an amount of 
chemical won'k done inconsistent with its limited powers and func- 
tions. The position and maximum elevation of the light curve 
of the spectrum as compared with the curves representing the 
ultra-red or dark heat waves, and the ultra-violet or actinic rays, 
indicate the same fact. The maximum elevation of the visible 
rays lies midway between the other two. That the ultra-red 
waves have no effect upon the receiving body of the retina, 
arises from several reasons; such rays are more varyingly and more 
wholly absorbed by the ever-present dust-clouds of the air, as 
also by all terrestial bodies, before the reflected rays reach the On Ileat. 
5 
eye; they are two powerful producers of molecular activity; they 
would too strongly upset the delicate equilibrium of the retinal 
temperature. Lastly, they are too irrefrangible to be bent by the 
weak ocular media, their inclusion with the other rays would 
lead to pronounced circles of diffusion. The ultra-violet rays 
are shut out for similar reasons: their great dispersion through 
their high refrangibility, by the two powerful atmospheres of 
the sun and the earth; because of their weak kinetic energies; be- 
cause of their too easy refrangibility by the media of the eye. 
These facts point to an action mediate between the strong 
effects of radiant heat or dark rays, and the weak effects of the 
chemical rays. The eye during its formative period had not 
only a limited range of wave lengths left at its disposal by the 
greedy forces of absorption and dispersion,1 but had out of these 
to choose such as were not too powerful or too weak, not too ob- 
stinate on the one hand or too willing on the other—too stub- 
bornly or too easily refrangible—for its delicate and limited 
powers and purposes. These chosen correspond to the uses and 
objects of the mind, which desires only intimations through their 
otherwise neutral instrumentality, of the sizes, distances and 
qualities of objects. All these classes of waves are, of course, of 
one and the same nature, absolutely identical in quality; we call 
one class heat-rays, because the molecules of all bodies vibrate 
responsively under the wave-strokes of these frequencies; the 
retina secretes molecules responsive only to the medium lengths, 
and these are called light waves in conseqnence; the chemical 
rays act on the haloid salts of silver as a reducing agent, and are 
called after their chemical or actinic effects. The qualitative 
differences called heat, light and chemical action are produced 
1. We realize how great is our loss by this power of atmospheric absorp- 
tion and dispersion, when we learn that the spectrum of the electric-arc 
in its ultra-violet region alone is from six to eight times as long as the 
whole visible spectrum, while that of the sun shine, owing to its absorp- 
tion by the two atmospheres, is very short. What our color-sense might 
have been had das Unbewnsste had these stimuli to choose from, is of 
course beyond our imagination, as well as beyond possibility. We could 
not have had these and the air too, though our air might have been freer 
from its several mile thick dust-shell. Even in its high altitudes the ultra- 
violet region of the spectrum is much brighter. 6 
L. W. Fox—G. M. Goula. 
by the receiving body, and result from the quantitative ones of 
the stimulus, from the varying kinetic energies of its different 
wave systems. Herein lies the physical origin of the shortness, 
less than one octave, of the “color-scale” as compared with the 
musical scale, a defect which is amply compensated for by the 
finer and richer perceptions of differences within the relatively 
short color octave. 
The term photo-chemical seems to be a general and large one, 
chosen, because of a certain analogy to the process taking place 
in the sensitive plate of the photographer’s camera. It is true 
that Boll and Kiihne succeeded in fixing an optogram in the 
eye of a rabbit, but as the macula contains no rhodopsin or vis- 
ual purple, it is considered that vision is inexplainable by the 
formation of optograms in the rhodopsin. The discovery of this 
substance which seemed to promise much in explaining vision, 
seems, indeed, to have left the subject exactly where it stood be- 
fore. 
In place of photochemical we humbly suggest the term thermal, 
and would endow the retinal end organs with an extreme sensi- 
bility to slight degrees and changes of heat, resulting from the 
impact of ether waves of varying lengths and frequencies. We 
state in advance the results we have reached by considering the 
retinal function as an intermediate between the objective stimu- 
lous and the cerebral result, and as not endowed with any spe- 
cific energy of its own: 
1. The retinal mechanism, in the first place, simply effects a 
pro-rata reduction of the highly delicate and infinitely divided 
kinetic energies of the ether-wave stimuli into such relatively 
finite quantities and differences of quantities, as alone can effect 
physiological or rather nerve-forces. It is a translator from the 
language of the imponderable and infinite (comparatively speak- 
ing) to that of the finite and known of neural vibrations. 
2. It moreover transfers these degrees of stimulation which it 
has rendered finite and recognizable, to the nerve-fibres whose 
function it is to convey them to the cerebral color and light pro- 
ducer. 
3. This process of reduction and transfer is through the medi- 
ation of molecular activities aroused in the retinal receptive or- On Heat. 
7 
gan by the stimulus, and perceived by the end-organs simply 
and only according to the height and degree of the aroused mo- 
lecular activity. 
4. The whole conception is based on these facts of physics: a. 
Ether waves can only impart their energies to the molecular con- 
stituents of matter, i. e., to bodies of like size or vibrating 
periods to those originating the waves; b. That the reception of 
such energy manifests itself primarily and always as increased 
vibratory activity of the receiving molecules; c. That our indi- 
cation, register, and name for increased molecular activity is heat. 
The essential nature of the transfer is a delicate perception of 
slight thermal differences. 
It is thus seen that this conception of the process is a total dis- 
carding of the fallacy of the old-time belief in specific energies of 
the end-organs, on which the theories of Young, Helmholtz and 
Hering rest. It is certainly time that this “physiological echo 
of the philosophy of Kant,” this endowing the end-organs or 
even the axis cylinders with specific powers and capacities, were 
given up. They are one and all only mechanical or proportional 
receivers and transmitters. It is beyond all question that it is in 
the brain that are eliminated the qualitative products called 
light, color, sound, etc., etc. The progress of the new psycho- 
logy in pushing back these phenomena to where they belong, and 
where all research proves them to arise, is slowly but certainly 
taking place. The confounding of the two functions has been 
from time immemorial; the old gods die hard, but their death is 
inevitable. 
We may briefly epitomize several concurrent facts and argu- 
ments in harmony with our view under the following heads: 
I.—Physical. 
Ethereal undulations are started into being by atomic or mole- 
cular blows upon the ether pressing about them, and this carries 
forward the vibrations, whose length or number per second, and 
whose height or amplitude, are identical with those of the strik- 
ing molecules. Now the science of spectrum analysis rests upon 
the fact that these ether waves can only set up energy in matter 
by imparting to its particles vibrations like those of the original 
starters of the ethereal oscillations. The receiving molecule 8 
L. W. Fox—G. M. Gould. 
must be consonant or attuned to receive them, or, in other 
words, they must be of the same relative sizes and densities, 
capable of the same vibrational frequencies or potential energies. 
Only those parts of a body that are small and slight enough to 
be moved by the correspondingly delicate and imponderable en- 
ergies of the ether wave can be affected by it. The ‘ ‘sympathetic” 
vibration of the piano string is set up by an aerial vibration started 
by another string whose vibratory period is consonant with that 
of the induced vibration of the second. But air-waves are pond- 
erable quantities, while the potencies of ether waves, while as 
certainly positive, are of such infinite smallness, that only 
bodies of like smallness can be affected by them. In no other 
way can their effects be realized upon matter. In this way alone 
can they be supposed to initiate chemical change; the increase of 
the molecular agitation under their rythmic strokes increases 
till the molecules are swung beyond the restraining forces of 
chemical cohesion, just as a huge pendulum may be set vibrating 
by the timely recurrence, at periods consonant with the pendu- 
lum’s beat, of a force of otherwise inappreciable delicacy. 
Spectrum analysis shows us the molecules of the sun’s atmos- 
phere, whose vibratory periods are consonant with those of the 
more heated, and so more actively vibrating molecules within 
his body, catching up the energies of the ether waves as they 
are passing outward past them, and thus causing the blank 
spaces or dark Fraunhofer lines of the sun’s spectrum. 
Now from the slowness of its propagation, and the powerfulness 
of its effect, we also perceive that the force we call nervous 
must be allied to the mechanical rather than to the atomic or im- 
ponderable order. It is a movement of masses rather than one 
of the atoms or molecules of masses. Since, therefore, ethereal 
cannot be changed into mechanical or nervous energy, except by 
the mediation of molecular energy, it follows that the primary 
retinal intermediate between ether waves and optic nerve tremors 
must be the increased molecular activity of some substance 
placed in or about the retinal end-organs. This increased molec- 
ular agitation we always call heat under any other circumstan- 
ces, and by that name alone do we generalize these phenomena. 
If it be asked how heat or molecular activity is transformed into On Ileat. 
9 
nerve-force, we can only say with the science of physiology, we 
do not know. 
The necessity of such an interposited substance may be shown 
in another way, from a simple consideration of the numbers of 
the ether-waves. Our inability to conceive or form any faint 
fancy of the tremendous number of vibrations striking the retina 
in a second, is only parallelled by the same inability to realize the 
minuteness of force conveyed by each individual blow. The 
number of waves per second is equal to the total number of 
people in four or live hundred thousand such worlds as ours. 
Such an approach to infinity is only equalled by the 
completed products of intellection we call the divine reason, 
and can not be an ingredient of any subsidiary process of physiol- 
ogy or sensation. Meynart estimates the cerebral cells re- 
maining for new impressions at 600,000,000. What have these 
to do with six hundred million millions ? It becomes evident 
from such numbers and minuteness, that to produce anything 
like physiological effects, and especially prompt ones, this in- 
finity must be brought within the limits of finite recognition, 
and processes. 
The thermal registration of such infinite molecular activities 
offers the means of such a pro rata reduction process, and we get 
a glimpse of the character of this reduction by consideration of 
the fact that a single spectrum color of apparently uniform tint 
to the eye is, in reality, formed at one extremity by waves differ- 
ing from those of the other extremity by so great a number as 
5 or 10 millions of millions per second. Blue, for example, seems the 
same to the eye whether made up of 651 or 637 million of millions 
of vibrations per second. Green may vary from 585 to 578; yellow 
from 521 to 516, red from 440 to 435. These are certainly 
sweeping reductions, for our minds are yet in no way fitted to 
conceive of such numbers as five or ten billions. The physiolog- 
ical process is thus seen to be a crude and heavy reaction com- 
pared to the unimagined delicacy of the impinging forces consti- 
tuting the stimulus. But with such massing together of numbers 
and forces we realize vividly that they are even yet too slight for- 
any thing except signs and indications. The energy of nerve trans- 
mission must be supplied by the self-contained forces of the or- 10 
L. W. Fox—G. M. Gould. 
ganism. The light is not transmuted into nerve force. The 
slight increments of force derived from the light waves is spent 
in setting up molecular activity in a delicately-poised and sensi- 
tive matter, which only serves as a releasing mechanism, a start- 
ing of the nervous discharge and regulating its degree, as the 
electric current may be too weak to notice or may set otf a dis- 
charge of tons of dynamite. 
It may be of interest to note in passing that this intercalation 
of a moleeular activity between the stimulus and the reaction ac- 
counts for the greater duration of time in the psychic (so-called) 
acts of sight, as compared with those of sound and touch-percep- 
tion. It is otherwise incomprehensible that touch should pro- 
duce psychic response sooner than the noble sense of sight, yet 
the relative periods are given as 0.167 seeond for sound, 0.213 
for touch and 0.222 for sight. The loss is easily explained, as 
the time necessary to superinduce molecular activity by the fra- 
gile ether-waves in the receiving substance of the retina. It is not 
improbable that the time lost in this reduction process was a 
great difficulty to overcome by evolution and natural selection. 
Mr. Romanes says of the eyes of the medusa3 that the interval 
between the stimulus and the response is so great that, were it 
any greater, the animal could hardly derive any benefit from 
their existence. 
II. —Physiological. 
Heat is therefore, not directly perceived by the nerves of the 
skin. That vibrational frequency of the ether we call heat, or 
sunlight, proceeding from molecular agitation of a “heated'” 
body, strikes the skin, but only affects its nerves by throwing 
the skin and matter about the nerve end-organs in the skin, into 
increased, or sympathetic vibratile energy. These vibrations, 
and not those of the sunlight or ether, are perceived or taken up 
by the nerve end-organs. 
Now there seems to be no reason or justification for the ten- 
dency of many minds to endow the different nerve systems of the 
body with utterly different natures and functions. Paul Bert 
grafted the tip end of a rat’s tail into its back, cut it off at its for- 
mer roots, and transmitted through it messages quite as well 
backward as they wereformerly carried in the reverse direction. On Heat. 
11 
The motor and sensory nerves may be made to exchange offices, 
and if “double conduction” is thus proved here, the principle of 
the uniformity of character of all nerve systems may be regarded 
as more than probable. The differences of result must consist 
primarily in the peculiar power and adaptability of the particular 
peripheral end organs to receive and transmute the peculiar 
stimulus into the common language of the nerves, into neural 
tremors, homogeneous in all respects except in the ever varying 
intensities or degrees of stimulation. The brain does the 
rest. 
The reception of lightwaves by the retina and their transmuta- 
tation into nerve messages by it must be of essentially the same 
nature as the reception of heat waves by the end-organs of the 
skin. Heat and light are different names for the same force. 
Both are one; both are ethereal undulations, though the slower 
are generally called heat, because they have a more pronounced 
sensational effect in that direction, yet light-waves are also and 
always heat waves, because all ethereal vibration is heat so long 
as heat is only definable as the energy of molecular or atomic 
motion, and so long as light-waves are the motions of ether. 
The logic is inexorable. 
Temperature, then, is the unerring register of molecular ac- 
tivity. Double the amount of molecular motion and the result- 
ant kinetic energy or heat, is proportionally increased. When 
the energy consequent upon any process is not guided by the en- 
vironment into any particular form, it invariably manifests it- 
self as heat; or in other words, when energy is not dissipated, it 
must be stored; and that storing is effected by increasing the 
molecular activity of the component parts of the body, or, as we 
commonly say, by augmenting the heat, or simply ‘ ‘heating” the 
body. We shall see later that the eye is jealously guarded 
against any dissipation of heat by its surrounding fat and capil- 
laries. 
We thus reach a point when we see that the retinal func- 
tion need not be carried so far in the extreme of activity as that 
implied in the word chemical, and that the character of that func- 
tion need not and cannot be essentially different from that of 
heat-perception. That the theory of the character of the retinal 12 
L. W. Fox—G. M. a(mid. 
intermediation as essentially thermal, may be acceptable, we have 
only to make the retinal end-organs much more delicate and sen- 
sitive to slight changes and degrees of temperature than the or- 
dinary sensory nerves of the skin. That with every degree of 
possible light-change there is also a temperature-change, there 
can be no doubt. Is such a refinement of heat-perception possi- 
ble? We see no inobviable objection to such a belief. If 
we consider for a moment the temperature sense of the skin, we 
can but marvel at its delicacy. The tips of the fingers can 
distinguish through their coarse and thick dermal covering differ- 
ences of so slight an amount as 0.2° to 0.16° R, and even to such 
extremes as that of 0.05° C., according to Lindemann. But in 
pathological conditions, and when the epidermis has been re- 
moved by burns, or vesicants, or herpes zoster, the exquisiteness 
of sensibility to temperature is not only immeasurable, but is un- 
believable by those who have not so suffered. 
But consider ow the peculiarities and adaptations of the eye 
in reference to this matter of thermal sensibility. If many of 
its arrangements do not answer to this purpose, it is as if they 
did. Teleological arguments seem to stare at one immediately. 
The thick, warm comforting of inexhaustible orbital fat, certainly 
points to preservation of warmth. It is indeed a “bed,” but it 
is certainly and more appositely a warm, bed. The extraordi- 
nary, almost excessive, development and closeness of the mesh- 
work of the chorio-capillaris, which wraps the whole retina about 
with its warm padding, also seems to correspond to the 
need for that high and uniform temperature, which would be de- 
manded both for a registration of slight thermal differences, and 
a reinstatement of the normal after each disturbance or stimula- 
tion. That the periphery of the retina perceives colors less 
easily than the center may be due to its more exposed and less 
blanketed condition. Its subnormal temperature results in de- 
creased sensitiveness. 
To prove this the authors have applied a pencil of ice to the 
exposed sclerotic of the converged eye and then thrown colored 
images obliquely upon the cooled retina. Under the influence 
of cold a brilliant color can quickly be reduced to black, and as 
quickly brought back to its normal by the removal of the On Heat. 
13 
ice, or more speedily by the application of a heated sponge in 
place of the same. The sensitiveness and quickness of response, 
even of the peripheral portion of the retina, to these changes 
seemed to us to point to an interference with a process more 
delicate than ordinary physiological ones, and which would cor- 
respond to the enormously fine degrees of molecular activity 
which, as we believe, lie at the basis of color distinctions. 
Our theory would suppose a highly sensitive and delicately 
poised receptive medium in a state of unstable equilibrium, nor- 
mally responsive to ether vibrations between the frequencies of 
392 millions of millions per second, and 757 millions of millions 
per second. Within these limits its molecules are varyingly and 
persistently capable of responding by their own vibrations to 
those of the impinging ether. The layer of pigment cells may 
be supposed to constitute or contain this receiving and 
leacting medium. Into this layer the long and blunt-pointed 
rods dip deeply, and doubtless serve as the transmitters of the 
raw material or nerve intensity corresponding to the sensation 
of simple light, or common day light, which is the most constant 
and pervasive of the stimuli, forming the background, as it were, 
of all light and color sensation. The delicately pointed cones 
do not extend into the substance so far, and by this fact and their 
shape and structure, plainly point to the function, more subtle 
and minute, of distinguishing those fine distinctions of difference, 
out of which color and tint are finally produced. This theory of 
the differentiation of function of the rods and cones, is borne out 
by many considerations, principal of which are these: The ab- 
sence of rods from the macula, where color definition gives greater 
precision to vision of small objects than simple light could do; the 
absence of cones in nocturnal animals, who do not require 
any color perception, etc. 
III. —Chromatological 
Sunlight is made up of the following proportions: 
Estimated Wave 
Approximate Wave 
Frequency. 
Parts 
Length. 
Bill, per Sec. 
64, Red 
140, Orange-Red 
6347 
80, Orange 
6094 14 
L. W. I1 ox—G. M. Gould. 
114, Orange-Yellow 
5900 
515. 
54, Yellow 
5713 
525. 
206, Greenish-Yellow 
5500 
535. 
121, Yellowish-Green 
5300 
550. 
134, Green and Blue-Green 
5100 
570. 
32, Cyan Blue 
4900 
600. 
40, Blue 
4760 
20, Ultra Marine and Blue Violet 
4500 
650. 
5, Violet 
4059 
700. 
According to Fraunhofer the comparative luminous values 
of the principal spectral lines and colors are as follows: 
Red 
B 
Orange 
C 
Reddish-Y ellow 
D 
Yellow 
D-E 
1000. 
Green 
E 
480. 
Blue-Green 
Blue 
31. 
Violet 
5.6 
From the last table we see that the curve of the light-produc- 
ing energy of the colors reaches its maximum elevation in yellow, 
or between yellow and green, dropping upon either side to zero 
in extreme red and violet. This point corresponds to the mean 
wave length, and also the mean wave frequency of all the colors 
producing sunlight according to the first table. We see that the 
quantities of light above and below this dividing line of green- 
ish yellow are about the same, so that the two parts of the spec- 
trum may be considered complementary to each other, and we 
likewise find that when one color is complementary to another 
their mean wave length, if the proportionate intensities of the 
components be also considered, falls near the average wave- 
length of sunlight white, i. e., near 0.00053-4 mm. We there- 
fore find the character of white changeless, however the inten- 
sity of illumination or quantity of light may vary. From the 
gray which is scarcely indistinguishable from black, on to the 
most dazzling brilliancy of the sun’s glare, it is simply a ques- 
tion of a preservation of the proportions of the compound, what- 
ever be the quantity of light. This is explained by considering 
the history of the eye. Diffuse daylight, since eyes began to be, 
has been their most persistent and continuous stimulus, over- 
whelming all other stimuli combined, in proportion and con- 
stancy. By day or night, whatever the circumstance, a combi- 
ation of the proportions of white light as given in the first table, On Heat. 
15 
has in ever varying strength been the stimulus of the eye. It is 
not. therefore, strange, but were it otherwise it would be so, 
that the eye has developed its most natural reaction and easiest 
response to such a stimulus: it is natural that the secretion, ar- 
rangement, and natural paths of the responsive retinal molecules 
should have become consonant with this peculiar and composite 
nature of the stimulus: it could not be otherwise. We conse- 
quently find that the subjective gray we perceive with closed and 
covered eyes (without pressure), and which is that low molecular 
activity arising from the retina’s own heat and pressure, is the 
feeble activity of the molecules in the same manner of response as 
to feeble objective stimuli of the weak daylight we call night. 
Subjective color vision is rarer than various strengths of achro- 
matic sensations. In traumatism vision is usually brilliant white, 
while by pressure on the ball and in cerebral disease all normal 
activity gives place to illogical mixtures of colors and of white in 
every degree and order. All this was to be supposed. 
Achromatic luminous sensation is, under all variations of in- 
tensity, a preservation of relative proportions of stimuli. Colors 
arise when the proportions are modified and changed in any way. 
But, in this connection, three important facts must be borne in 
mind. 1. Color-perception cannot arise till a certain elevation of 
molecular activity has been reached in the retina, until either the 
color-producing waves have a certain strength of themselves, or 
until a general illumination by (weak) white has taken place. We 
should suppose a priori, that a certain elevation of molecular ac- 
tivity would be a prerequisite of a perception of changes in pro- 
portion. 2. All colors appear most saturated or pure at that de- 
gree of illumination we call diffuse daylight. This also was to be 
expected from the history of the eye, and from the supposition 
of our theory. The eye would, of course, learn best to perceive 
a change of proportions when acting under its most continuous 
and normal stimulus. The reaction to diffuse daylight being 
the normal and healthy degree of retinal molecular activity, it 
judges most accurately any departures from that standard pre- 
cisely at the full of its normal response. 3. But increase the in- 
tensity of the stimulus producing color, and color is quickly 
remerged into neutral and brilliant white, which, with still in- 
creased intensity, changes in quantity but not in quality. 16 
L. W. Fox—G. M. Gould. 
On any theory based upon the supposition of specific ener- 
gies, these protean changes of intensity into quality and quality 
again back into intensity, must be inexplainable, twist and turn 
it however one please. But, if all these differences of quality 
are relegated to the differentiating center—of the brain—and the 
retinal mechanism be considered a responsive apparatus, always 
consonantly registering the height to which the molecular activ- 
ity is aroused by the stimulus (which at last varies only in de- 
gree or intensity)—if we so face our facts about, light falls where 
before was mystery. 
Another result of experiment is that different spectral colors 
require different periods of impact to bring forth the resultant 
color-sensation. On the supposition of specific energies why 
should a green nerve take a longer time to react than a blue one, 
and why either a longer than a red one ? But, if green is produced 
only by a higher degree of activity of the retinal molecules, 
blue by lower intensity, and red by a still lower, then clearness 
again takes the place of darkness. 
Another related fact also linds its explanation in the same way: 
that color and tint change by continuous observation. If con- 
tinuously observed, pure colors may be merged into white; the 
whole spectrum may, in this way, become dichromic, blue and 
red alone remaining with a white interspace, or even these may 
fade into achromatic luminosity. This, in our view, bespeaks a 
cumulative effect of the repeated ether blows, which increase the 
retinal activity beyond the point of normal differentiation of 
color difference. The apparently correlated fact that after long- 
wearing of colored glasses, for example, blue, one is unable to 
distinguish blue colors, we think is due to cerebral rather than 
retinal fatigue. 
After-images logically follow from the theory. That the retinal 
activity of the molecules must continue for a period after the cessa- 
tion of the stimulus, would be a simple result of the laws of phy- 
sics. As the molecular activity falls to the sensation of relative 
black, the sensations run through the scale of colors correspond- 
ing to the different levels of molecular agitation successively 
reached. It is especially consonant with our theory that the 
purest colored after-images are seen only after the extremest ac- On Heat. 
17 
tivity has been set up by brilliant white, and that no after-image 
is ever of a color of higher intensity than the causing stimulus; 
a stream cannot rise higher than its source. 
Students acquainted with the heterogeneous mass of facts in- 
cluded under chromatological optics will, at once, see the range 
of applicability the theory has, but the whole field is in such an 
untilled and disorderly state that we forego further application 
for the present. 
IV. —Psychologic a l. 
It may be said this view of the subject only pushes the mys- 
tery farther back, only relegates to the inscrutable processes of 
the coordinating center the real problem to be explained. To 
this accusation we shall only be too glad to plead guilty. If 
that much of definiteness could be reached, it would be well. It 
is far better to know just where the mystery is than to know not 
where it is; likewise it is better to know where it is not, than to 
know that it is everywhere. We are not vain enough to suppose 
that we have done more than to localize the mystery, to push it 
a step or or two farther back, and indicate the direction in 
which investigation might be prosecuted to bring the function of 
the retina into understandable processes analogous to those of 
other sensory organs. How, at last, the brain or the mind 
creates color and light from the raw material of neural tremors 
not unlike those of other nerve systems from which it eliminates 
very different products; how it makes of the one light, of another 
sound, of yet others, heat, muscular sensation, odor, taste—all 
this no witling would, at the present standpoint of science, at- 
tempt to intimate. It is not too much to say, no human intelli 
gence has the faintest glimpse of an idea of the method. That 
the creation of light and color is absolutely and purely cerebral, 
there can be not the slightest hesitancy in affirming. Among a 
hundred other proofs is this striking one that two comple- 
mentary colors thrown independently on identical spots of the 
two retinae are (barring certain conditions and only apparent ex- 
ceptions) perceived as the homogeneous resultant, white, by the 
fusing and unifying power of the mind. How the nerves trans- 
mit their messages is likewise mystery, but how they transmit 
degrees and differences of temperature is less a mystery than how 18 
L. W. Fox— G. M. Gould. 
they might transmit colors already formed, and of infinite variety. 
It is a plain fact that fine gradations and changes of thermal differ- 
ences are recognizable and transmissible by them; and it is as 
certain that the only endowment we need to give them to ex- 
plain their function from our stand point, is that of heightening 
and rendering their natural power of transmitting degrees of 
stimulus more delicate and precise. The simple fact of the spec- 
ification of this or that end-organ as stimulated, the location of 
the position stimulated by the message of the connecting nerve- 
thread, is the basis of the fruitful theory of local signs, where- 
by the mind, by habit and comparison, learns of the direction 
and position of the external object. By the power of the end- 
organs and of the nerves to perceive and transmit the exquisite 
differences of thermal stimulation, the mind likewise elaborates 
corresponding states of feeling we call color and light, as the 
psychical analogues of the variations of ethereal wave-length, 
frequency and amplitude. 
V. —Pathological. 
The semi-pathological facts and condition of color-blindness 
are often considered to be the test and touchstone of theories of 
color perception. How do we square our account ? Our de- 
mands are once more of the most praiseworthy modesty; we 
simply “await the time in patience” till something more is 
learned, if it be possible, of the anatomical causes. Color-blind- 
ness may turnout to be either a result of retinal deficiency, or 
it may be purely cerebral, or both. If it is retinal it must cer- 
tainly appear in time whenever an eye markedly color-blind, is 
secured for microscopical examination. So far this seems not to 
have been done, though there are some 1000 color-blind men in 
every million about us. One feels like ticketing his color-blind 
friend, or, perhaps, like putting to him the quizzical though 
pointed query of the medical student to the young lady with a 
remarkable peculiarity of the elbow-joint, “Where does your 
family bury ?” 
The differentiation of function, whereby the cones have be- 
come the intermediates of the more strictly color sensation, is, 
as we have observed, borne out in proof by the facts of com- 
parative anatomy, and by many other more general considera- On Ileat. 
19 
tions. Disuse is followed by atrophy, and if color-blindness is 
retinal, that is, if it is caused by the malformation, pathological 
condition, or absence of the cones, such facts must appear under 
the microscope. Whether some of our ancestors were, for ex- 
ample, the cave-men, and more particularly some household 
slaves or drudges of this tribe, who were never permitted to go 
forth by day—to whom, by the subtle laws of atavism, our color- 
blind neighbors hark back for their peculiarity—all this is, as 
yet, in the dim domain of speculation. More probably the 
color-blind, if we may again beg their pardon for the second ca- 
lumniating inference, might go back to the night-howling apes 
in their genealogical tree. Unfortunately, we have as yet been 
unable to catch any of the present descendants of the nightr 
howlers, however prevalent they are in modern civilization, and 
however gladly we would like to do so, in order to more fully 
investigate in the laboratory whether the two characteristics of 
color-blindness and nocturnal bellowing still go hand in hand ! 
Of course it is possible that the retinal defect, if such it be, 
may also reside in the pigment layer where the molecules, sym- 
pathetically responsive to vibrations of certain rapidities are 
wanting or deficient, or, from any imaginable cause, fail in func- 
tional activity. 
It is more difficult to suppose the seat of the defect to be in 
faulty neural transmissson, yet even this might be. In either 
of the latter cases the morbid histological anatomy would not 
be shown by the microscope, certainly not in the former of the 
two. 
We incline to think the locus of the functional failure will 
eventually be found to be cerebral; in which case any present at- 
tempt to reason about it would be quite as profitable as to dis- 
cuss the famous Germanic question: Utmm chimcera bombinans 
in vacuo posset comedere secundas intentiones f It would be just 
as wise to repeat with old Porterfield the quaint, “It is the will 
of God,” and there let the matter rest. 
Resume. 
Ethereal vibrations can only impart their energy to molecules 
of a like minuteness to those originating the vibratile energy. 
The instant and constant result of such imparted energy is the 20 
L. W l1 ox—G. M. Gould. 
increase of molecular activity in the receiving body. Our name 
for that increased molecular activity, however minute and slight 
it be, is heat, which is the precise register of it in all ways and 
degrees. The retinal end-organs are not essentially different in 
function from those of the nerve-end organs of the temperature 
in other parts of the body, only more refined, more exposed, 
more percipient of the exquisiteness of change. The pigmen- 
tary layer is the seat of the molecules, suspended in delicate 
poise of unstable equilibrium, whose function it is to respond 
sympathetically to the impinging ether waves, and whose an- 
swering vibratory activity is perceived by the rod-tips as the 
achromatic stimulation of white, or common day-light; and by 
the more delicately pointed and retracted cone-tips as those finer 
gradations of vibrational frequency, finally resulting in the cere- 
bral product denominated color. The optic nerve tremors or 
transmissions are not unlike those of other nerve systems which 
transmit delicate differences and amplitudes of stimulus, and 
from which raw material, the cerebral artificer fashions the mar- 
vellous products, light and color with infinite cunning and by in- 
scrutable methods. 
That the feeling of light or color is specifically and utterly dif- 
ferent from that of heat, gives us no pause or trouble. The 
whole range of retinal temperature change, within which and 
by which the infinitude of color-change must play, is probably 
far less than that of a single thermometric degree. That also 
creates no questioning astonishment in us. Students of physi- 
cal optics are struck at every point of their investigation with 
the inexpressible complexity and infinite detail harmoniously re- 
sultant from the simplest and most single of causes. We, more- 
over, acknowledge the homeliness and simplicity of the present 
theory. The plain and barren phenomena of heat sensation 
seem, at first sight, to be beyond reach of analogy or likeness to 
those brilliant results of astounding subtlety and beauty, which 
we call light and color. But let us have a care of scorning sim- 
ple and near-at-hand causes. It is by such, as we have at last 
learned, that Nature produces her most dazzling, most complex 
and far distant aims and results. The day of heaven-topping 
theories is happily past. In science as also in metaphysics, we On Heat. 
21 
are fast learning that the poet’s verse is as well justified as in 
matters of every day life: 
“Our best things are nearest us, 
Lie close about our feet; 
It is the distant and the dim, 
That we are fain to greet.” 
To be acceptable to the cultivated inductive mind of this age, 
a theory must not contradict the evident laws of physics; it must 
not contradict the evident facts which it is seeking to explain; it 
must explain the unknown by analogy with the laws of the 
known; it must have the crowning merit of unity or simplicity, 
whence multiplicity of result proceeds logically from singleness 
of cause; it must, lastly, more properly localize the mystery yet 
remaining, and say ‘‘Here is light, there it is not.” We mod- 
estly submit that previous theories, with the notable exception 
of that of our leader, Wundt, do not answer to these demands 
of the scientific spirit, and that our own, in a more or less per- 
fect manner, does do so. 
Postscript. 
Since the above article was written we have stumbled upon an 
article, previously overlooked, by Dr. Burnett, printed in the 
July, 1884, number of the American Journal of the Medical Sci- 
ences. This accomplished writer takes essentially the same 
ground as ourselves in regard to the fundamental nature of the 
retinal process. We do not wish ‘‘maledictions on him who dis- 
covers our good things before we did.” 
The most cursory examination of our articles will show that, 
though we arrive at the same result, it is by entirely indepen- 
dent roads and through a different country. And so, though at 
the time of writing, we had no knowledge that any other had 
struck upon the same thought, it gives us all the more confidence 
in the security of the result to find such a powerful ally. 
Dr. Burnett’s reference to Preyer’s article in Pflilger’s Archiv, 
sends us all to Preyer as the real author of the conception that 
the retinal function is a refined temperature-sense, though so far 
back as 1877 Pfliiger had stated his belief in the phylogenetischer 
Zusammengehcerigheit des MYcerme-und Licht-sinns. Of this, how- 
ever, Preyer was as ignorant as we of Burnett’s article. 22 
L. W. tox—G. M. Gould. 
It is a pity that Preyer should have adopted the semi-exploded 
doctrine of specific energies of the retina. His division of two 
neighboring axis cylinders each into two cones, each of which 
separately perceives only one of the four principal colors, each 
set uniting a “cold” and a “warm” color, is quite as perplexing 
as Young’s three-fold, or Hering’s triplicate polar divisions. The 
many analogies he draws between vision and the dermal tem- 
perature or tactile senses seem to us forced and beside the mark. 
In brief, there seemed little in it to repay the difficulty we had 
in securing its perusal. 
On the other hand we were pleased to find Dr. Burnett throw- 
ing aside the whole machinery of specific energies, together 
with the theories of Young and Hering, built upon it. 
We wish that the argument from embryology A cre as “conclu- 
sive” as Dr. Burnett inclines to think it. It is not quite true 
that the eye in vertebrates is formed from the skin and grows 
inwards to the brain. It would be true, if we could consider the 
sclerotic and the humors as the eye; but, functionally, these are 
only its adjuncts. The eye is par excellence the retina, and in- 
stead of this all-important organ growing inward from any der- 
mal modifications it is found, embryologically, that it grows out- 
ward from the brain, and joins the epidermic helps. The greatly 
impressive fact is that the brain comes out to see, and that it 
comes the whole distance, and not that the skin goes in to tell 
the brain what it has undergone. Vision is of all the senses the 
most purely cerebral, or psychical in character. It is true that 
the argument may be rescued by Prof. Ray Lancaster’s ingeni- 
ous, interesting, but improbable supposition that the eye of the 
Ascidian larva, supposed to be the parent of all vertebrates, was 
situated on its brain, the rays of light penetrating with ease its 
transparent body. 
Or, the fact might be emphasized that brain and skin are both 
differentiations of the external larval layer or epiblast, so that, 
in a sense, nerve-center and nerve-terminal, inner and outer, 
mind and matter are all one ! But in this way we could, of 
course, go back to the original undifferentiated protoplasmic 
cell. 
In a word, the eye’s temperature-sense will bear out no such On Heat. 
23 
literal analogies to the skin’s functions as Preyer would find, and 
the embryological argument would prove. The division of 
function was too far back; it must, as Genesis shows, have been 
the very next day after the “greater light” was placed in the 
heavens. About the only certain analogy which we can to-day 
draw between the two, is the very general one we have insisted 
on, of the perception of differences of molecular activities. But 
a cone with the pigment cells in front, and the complex machin- 
ery of numerous, not understood retinal layers behind, is a very 
different organ from the dermal corpuscles or end-bulbs of Pa- 
cini, Wagner, Krause or Merkel, and in the present state of our 
knowledge close analogies are drawn at a great hazard.