206 PHOTOMICROGRAPHY 



of the exposures through the three screens is all the 

 compensation needed for the different constitution of the 

 light from various sources. Mark the plates A, B, C 

 with pencil on the corner of the film before development 

 to correspond with the particular taking screen used. 



To ensure equality of density in all three negatives 

 the three plates must be developed at the same time in 

 the same dish after accurately timed exposures. Develop- 

 ment must be carried out in darkness for the proper 

 time, depending on the developer and temperature ; 

 suitable times are usually given in the plate box for a 

 definite developer. The light given by the safelight, 

 allowable with the panchromatic plates, is not sufficient 

 to enable a correct judgment of the density to be made 

 by visual examination. 



Three positive transparencies are made on bichromated 

 gelatine on a celluloid base from the three negatives. 

 Special celluloid films coated with gelatine and silver 

 bromide emulsion can be purchased from Sanger-Shep- 

 herd. They may be obtained ready sensitised, but it is 

 better to sensitise the films for oneself. To do this the 

 film is placed in a solution of ammonium bichromate 

 (p. 273) and bathed for about five minutes, and after- 

 wards dried in the dark. 



The films are then placed with the shiny celluloid 

 surface (not the sensitive side) next the negative and 

 printed like P.O. P., using an actinometer to get correct 

 depth of printing. A suitable guide to the time of 

 printing is obtained by printing a piece of P.O. P. under 

 a negative of similar density side by side with the colour 

 negative, and removing the film when the P.O. P. is 

 printed to a depth somewhat lighter than is usual for 

 that type of print. Each film must be marked A, B, or C 

 to correspond with the negative from which it was printed. 



After printing, the films are developed by immersion 

 in warm water (about 100 F.) until the parts of the 



