September 30, 1960

Dr. Peter Walker
Department of Zoology
West Mains Road
University of Edinburgh
Edinburgh 9, Scotland

Dear Doctor Walker:

My thanks for the reprints and for your letter of July 19th, all of
which | have been digesting over the summer, | do appreciate your offer of
help and hope to seek your advice, especially as our project seems to be con-
verging even closer to your own recent technical Interests. During the past
couple of months, my own views as to the best direction of the program have
altered slightly - some of the reasons for this are summarized in the
accompanying reports. {§ am happy to send these to you on a personal basis;
there is nothing officlally restricted In them but | would be rather
embarrassed if they were published without careful review. The main point
Is thet the most useful aspect of ultraviolet would, of course, be a
spectrum and not merely an absorption image at a single wave length. In
fact, it Is a reasonable supposition that any object that can be seen under
phase contrast would also scatter sufficiently to give some sort of image
in the ultraviolet. In the end we would still have to make at least a
rudimentary comparison of absorption at two or more wavelengths. On the
other hand, It seems much simpler to drive one's image Information by .
phase contrast microscopy, or perhaps Interference microscopy, and to con-
centrate on using the ultraviolet for spectral rather than image purposes.
This would ental! setting up a microspectrophotometer which could rapidly
scan individual particles as they passed the aperture; those particles which
gave an interesting spectrum, I.e. which have a zero in the first or second
derivatives of absorption with respect to wave length, might then be photo-
graphed and the signal for the image and the associated spectrum telemetered
back to earth. Casperson thinks that with the new lenses, It might be just
possible to make this scheme work, though there may be formidable difficulties
on account of energy limitations and scattering by the particle. Your own
comments on this would be appreciated. { had In mind that we might look Into
the possibility of using a quartz fiber terminating In a tip of just under
one micron diameter as the condenser and collecting al! of the light which
passed through the specimen as far as this Is possible with the available
optics. | have not made any calculations as yet on the plausible energy
values available from existing light sources and the sensitivity of useful

‘cf (MAHON
Or. Walker, cont. September 30, 1960

photodetectors, factors which obviously would control the scanning rate.
For this to be useful, we should have a quite rapid scan.

AS one approach to the depths of the problem, | thought we might look
Into rapid scanning for UV spectra on a macro scale but there seems to have
been more work In the Infrared than In other regions. Do you know of the
existing literature on such Instrumentation? Are you acquainted with any
commercially avallable Instruments for spectral display on the CRO? 1!
understand that the American Optical Company has done something along these
lines for the visible spectra and | am writing them accordingly.

It might seem to you that these plans are premature In relation to
realistic expectations for planetary flight. As you probably know, the
U.S. planetary program will be based on the Saturn vehicles whose develop-
mental flights are scheduled for 1964. It would be an additional few years
before we might realistically hope to plant one of these experiments on Mars.
However, there 1s a tremendous amount of lead time required for the mutual
adaptation of the space craft and the instruments; consequently our margins
of time are not comfortable at all. This spectromIcroscope project is
intended as the work horse of these definitive flights; we are also working
on some less ambitious techniques as @ back stop and also for possible use
of earlier missions with avallable vehicles.

Are you likely to find yourself fn this country at any time during the
next year? If so, | would welcome the opportunity that it might afford for
us to get together. Falling this, |! will probably be coming over next April
and if there is any possibility of ft, 1 would be very pleased to take the
opportunity to visit you (and of course, my many other friends) at Edinburgh.

Yours sincerely,

Joshua Lederberg
Professor of Genetics

P.S. There Is one particular thing you could probably do for me at much
less effort than It would take me: a complete listing of specific refer-
ences to ultraviolet spectra of cells and other particles obtained by
photoelectric recording and through a microscope.

Ewe. 7P7, Exo & £-42.,