A METHOD OF MEASURING THE ABSOLUTE SENSITIVENESS OF PHOTOGRAPHIC DRY
PLATES.

[Footnote: From the Proceedings of the Academy of Arts and
Sciences.--_Amer. Jour._]

By WILLIAM H. PICKERING.


Within the last few years the subject of dry plate photography has
Increased very rapidly, not only in general popularity, but also in
importance in regard to its applications to other departments of science.
Numerous plate manufacturers have sprung up in this country as well as
abroad, and each naturally claims all the good qualities for his own
plates. It therefore seemed desirable that some tests should be made
which would determine definitely the validity of these claims, and that
they should be made in such a manner that other persons using instruments
similarly constructed would be able to obtain the same results.

Perhaps the most important tests needed are in regard to the
sensitiveness of the plates. Most plate makers use the wet plates as
their standard, giving the sensitiveness of the dry plates at from two to
sixty times greater; but as wet plates vary quite as much as dry ones,
depending on the collodion, condition of the bath, etc., this system is
very unsatisfactory. Another method, employed largely in England, depends
on the use of the Warnerke sensitometer. In this instrument the light
from a tablet coated with luminous paint just after being exposed to a
magnesium light is permitted to shine through a colored transparent film
of graduated density upon the plate to be tested. Each degree on the film
has a number, and, after a given exposure, the last number photographed