MAMMALIAN. 69 



Major Russell's tannin process, is in many respects very well adapted for the 

 purposes of microscopic photography ; by modifying the collodion and developer 

 we may obtain almost any desired result, and I sometimes employed it, though not 

 so much as I undoubtedly should, had I known earlier how to control the strong 

 tendency to solarization and thinness, of negatives obtained by the tannin process 

 by means of direct smilight reflected through a low objective. This control is best 

 attained by using a large proportion of bromide in the collodion, in order to diminish 

 the excessive intensity and hardness of the tannin negative, and a very large propor- 

 tion of citric acid in the developer. 



The formida for collodion which seemed to me to give the best residts Avith the 

 dry process was as follows : — 



Bromide of Cadmium ....... 40 grains. 



Iodide of Ammonium ....... 22 " 



Collodion ......... 8 ounces. 



The plates were prepared as usual, and immersed in a solution of tannin (15-20 

 grains to the ounce). 



The developer is made from the two following solutions : — 



No. 1. — Pyrogallic Acid ........ 12 grains. 



Alcohol ......... 1 ounce. 



No. 2. — Nitrate of Silver 20 grains. 



Citric Acid 120-180 " 



Water ......... 1 ounce. 



Fifty minims of No. 1 are diluted with two ounces of water, and a few drops of 

 No. 2 added to the quantity necessary to develop a plate. The resulting pictures 

 are very fuU of detail, and the great convenience of the dry process wiU not fail to 

 be a very strong recommendation to the microscopist, who may by this means have 

 a stock of sensitive plates on hand ready for use at any moment. Figs. 30, 31, and 

 32 were taken by the dry process, and are certainly not surpassed in detail and 

 delicacy by any of the others. 



