46 The Microscope 



between the lens and the illuminant. Some enlargers have no condenser, 

 which has the effect, desired by a few pictorial photographers, of soften- 

 ing the image. 



Processing Negatives. Processing negatives consists of five steps. These 

 are development, hardening, fixing, washing, and drying. 



Development. The film to be processed may be either in the form of 

 a roll or a sheet. The former is invariably, and the latter best, processed 

 in a tank. Tanks for rolls of films consist of a lightproof case within which 

 there is some device that allows the film to be spirally coiled without 

 touching itself. Any dealer will demonstrate a variety of these tanks. 

 Roll-film tanks have some kind of light-trapped aperture that permits the 

 necessary reagents to be poured in and out in daylight so that the dark- 

 room is required only for loading the tank. 



Sheets of film are usually processed in rectangular, open-top tanks and 

 must therefore be kept in total darkness throughout the whole of develop- 

 ment and hardening and the early stages of fixation. Films handled by 

 this method are placed in rectangular metal frames, from the top of 

 which horizontal arms project to rest on the sides of the developing tank. 



The reagent used for developing the latent image is an alkaline re- 

 ducing agent. The grain size of the film, on which resolution depends, 

 can be increased during development by using solutions that are too 

 strongly alkaline. Most "fine-grain" developers are solutions of p-diamino- 

 phenol, or some similar compound, buffered with borates in a solution 

 of sodium sulfite that delays the spontaneous decomposition of the de- 

 veloper. The selection of the particular fluid to be used should be left 

 strictly in the hands of the manufacturer of the film, who usually supplies 

 either the formula or the reagent in ready-to-mix form. Experimenting 

 with fancy developers is not recommended to the producer of technical 

 photomicro graphs . 



Not only should the formula recommendations of the manufacturer 

 be rigidly followed but also his specifications as to time and temperature. 

 The photomicrographer who always uses the same film developed in the 

 same developer for the same time at the same temperature can be certain 

 that variations in his negatives are caused by faulty exposure, selection of 

 the wrong filter, or imperfections in his optical system. None of these 

 things can be corrected in the course of development, they can only be 

 made worse; without standardized development they are difficult to 

 detect. 



Most film-processing instructions offer times for "tray" development 

 as well as tank. High-resolution photomicrographs are much better tank- 

 developed. Tanks containing roll film, unless the manufacturer's direc- 

 tions read to the contrary, should be inverted about once a minute during 

 the course of development. Cut film hanging in tanks should be removed 



