of ascertaining the two Gauss points of a
bi-convex lens, and a sheet exhibited in which the various kinds of
lenses with their optical centers and Gauss points were shown. For this
drawing he (Mr. Taylor) said he was indebted to Dr. Hugo Schroeder, now
with the firm of Ross & Co. The lecturer congratulated the
newly-proposed member of the Society, Mr. John Stuart, for his
enterprise in securing for this country a man of such profound
acquirements. The subject of distortion was next treated of, and the
manner in which the idea of a non distorting doublet could be evolved
from a single bi-convex lens by division into two plano-convex lenses
with a central diaphragm was shown. The influence of density of glass
was illustrated by a description of the doublet of Steinheil, the parent
of the large family of rapid doublets now known under various names. The
effect of thickness of lenses was shown by a diagram of the ingenious
method of Mr. F. Wenham, who had long ago by this means corrected
spherical aberration in microscopic objective. The construction of
portrait lenses was next gone into, the influence of the negative
element of the back lens being especially noted. A method was then
referred to of making a rapid portrait lens cover a very large angle by
pivoting at its optical center and traversing the plate in the manner of
the pantoscopic camera. The lecturer concluded by requesting a careful
examination of the valuable exhibits upon the table, kindly lent for the
occasion by Messrs. Ross & Co.

* * * * *

IMPROVED DEVELOPERS FOR GELATINE PLATES.

By Dr. Eder.

We are indebted to Chas. Ehrmann, Esq., for the improved formulas given
below as translated by him for the _Photographic Times_.

Dr. Eder has for a considerable time directed especial attention to the
soda and potash developers, either of which seems to offer certain
advantages over the ammoniacal pyrogallol. This advantage becomes
particularly apparent with emulsions prepared with ammonia, which
frequently show with ammoniacal developer green or red fog, or a fog of
clayish color by reflected, and of pale purple by transmitted light.
Ferrous oxalate works quite well with plates of that kind; so do soda
and potassa developers.

For soda developers, Eder uses a solution of 10 parts of pure
crystallized soda in 100 parts of water. For use, 100 c.c. of this
solution are mixed with 6 c.c. of a pyrogallic solution of 1:1