March 13, 1961

Or. J. W. Mitchell
Department of Physics
University of Virginia
Charlottesville, Virginia

Dear Doctor Mitchell:

May { ask your help to bring me up-to-date on a point of literature
that | hope will be as famillar to you as to anyone. tn thelr well known
paper in the Zeitschrift f. Physik tn 1930, Hi Isch and Pohl refer to the
darkening of silver chloride crystals. They also refer specifically to the
selective bleaching In a band corresponding to the monochromatic radiation
used for the Illumination of the crystals. They and subsequently, Or. Fred
Seitz, have attributed this selective bleaching to the photo-excitation of
coltoid resonators. | have been Interested in this phenomenon primarily as
a conceivable means of producing narrow band filters or more precisely,
spectral wedges for use in spectrophotometric devices. The particular region
that | am Interested In myself Is the ultraviolet between 250 and 300 nm and
it Is, of course, quite uncertain whether the Hilsch and Poh! phenomenon
would be directly useful. However, as far as 1 can find, it Is the only
material manifestation of the principle of selective bleaching that would
be one approach to the preparation of such filters.

As far as 1 can determine, although the work of HiIsch and Poh! has
been widely quoted, no one has made any particular effort to repeat or
improve upon their experiments. This is the main question | would like to
ask you, namely, whether you know of such a repetition. 1! would also
appreciate your views on the correctness of the interpretation of selective
bleaching. tt has occurred to me, as a rather far fetched possibility, that
Hi isch and Poh! may Instead have been observing an Interference pattern of
stiver deposition In the crystals such as forms the basis of the old Lipmann
process of color photography.

While | am on that subject, | might remark that | have heard rumors
that the Lipmann Interference prints have been adapted for the production
of practical filters, also, 1 have not myself been able to track down any
recent work on this. If any such has come to your attention, {| would be most
grateful as 1! would for your helping In dealing with the above questions.

Yours sinceraly,

Joshua Lederberg
Professor of Genetics

“me ro HOF4!W