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 1:10, without the addition of any bromide.

More pleasant to work with is Dr. Stolze’s potassa developer.  No. 1:  Water, 200 c.c.; chem. pure potassium carbonate, 90 gr.; sodium sulphite, 25 gr.  No. 2:  Water 100 c.c.; citric, 11/2 gr.; sodium sulphite, 25 gr.; pyrogallol., 12 gr.  Solution No. 2 is for its better keeping qualities preferable to Dr. Stolze’s solution.[A] The solutions when in well stoppered bottles keep well for some time.  To develop, mix 100 c.c. of water with 40 min. of No. 1 and 50 min. of No. 2.  The picture appears quickly and more vigorously than with iron oxalate.  If it is desirable to decrease the density of the negatives, double the quantity of water.  The negatives have a greenish brown to olive-green tone.  A very fine grayish-black can be obtained by using a strong alum bath between developing and fixing.  The same bath after fixing does not act as effectual in producing the desired tone.  A bath of equal volumes of saturated solutions of alum and ferrous sulphate gives the negative a deep olive-brown color and an extraordinary intensity, which excludes all possible necessities of an after intensification.