38 ROBERT M. YERKES AND JOHN B. WATSON 



Directions for making up gelatine or collodion filters from 

 approved formulae may be found in works on photographic 

 methods. ' 



The essential points of method may be stated. Lantern dry 

 plates should be (i) fixed in a solution of hyposulphite of soda, 

 (2) cleared in a ferricyanide and hyposulphite solution, if neces- 

 sary, (3) washed thoroughly, and (4) immersed in the solution 

 of the desired stain until the proper depth of color has been 

 attained. 



For the following practical and more specific directions, the 

 writers are indebted to Mr. Willard Greene. 



Select a heavily coated dry plate, fix, clear (if necessary) as 

 above, wash for an hour in running water, and dry. Having 

 prepared the dye solution according to formula, pour it into a 

 tray about the size of the plate to be stained. Place one edge 

 of the dry, or partially dry, plate in the solution and by tipping 

 the tray, and at the same time lowering the plate, cause the 

 solution to fiow over it in an even wave. Leave the plate in the 

 dye until it has attained proper depth of color. The time may 

 be accurately determined by trial. Upon removal of the plate 

 from the dye, rinse it in running water. Drain and then wipe 

 lightly in both directions with soft surgical gauze to remove 

 excess of solution. After a few minutes wipe again to remove 

 surface moisture and then place on rack to dry. 



When skill has been achieved in staining plates to the proper 

 depth, a filter of more satisfactory quality may be made by 

 coating optical glass with a carefully filtered and clarified solu- 

 tion of gelatine or collodion, and, when dry, immersing in the 

 ■dye-solution for the proper length of time. Gelatine filters may 

 be protected by binding plates of clear glass against them. 



Solutions. — (Wet filters) Aqueous or alcoholic solutions of 

 ■dyes and other substances exhibit selective absorption (the 

 capacity to transmit only light of certain wave-lengths). The 

 quality and intensity of the light transmitted depends upon the 

 concentration of the solution and the thickness of the layer. 

 Either singly or in series, layers of solutions which exhibit 

 selective absorption may be used as ray filters. The absorption 



' Bolas, T., Tallent, A. A. K., and Senior, E. A handliook of photography in 

 colors. New York and Chicago, 1900. Chapter 21. " The manufacture of color 

 filters," pp. 157-lGO. 



