220 PHOTOMICROGRAPHY 



Isolar and Paragon plates. It dissolves out during 

 fixation and forms a very clean backing. 



Developers. There is a very long list of developers from 

 which to choose, and their characteristics vary to some 

 extent. There are quick- and slow-acting developers, 

 developers that tend to give a yellow negative, and others 

 that give a grey-black image. Some are greatly affected 

 by the temperature of development, while others are not 

 so sensitive in this respect, and so on. A few reliable 

 formulae are given in the appendix, but the inferiority 

 of others is in no way implied. Suitable formulae for most 

 developing agents are to be found in such useful books of 

 reference as the British Journal Photographic Almanac 

 and The Photographic Annual, and practically any of 

 them, if rightly used, will give first-rate negatives for 

 photomicrographs. It is necessary, however, to know 

 the characteristics of whatever developer is selected, 

 and to use it with judgment in order to obtain a negative 

 of the desired quality. Some developers tend to give 

 negatives with great contrasts, while others more readily 

 give softer results, but an equally good negative could 

 be obtained with either sort if used property. 



Developers do not, as a rule, keep long in dilute 

 solutions, and for intermittent use are most conveniently 

 obtained in tabloid form and dissolved as required, or 

 kept in concentrated solutions and diluted for use. Of 

 the latter type there are many on the market containing, 

 as a rule, para-amidophenol and a caustic alkali, such as 

 Rodin al, Azol, Victol, Certinal, etc., and they are par- 

 ticularly useful, serving either for plates, papers, or 

 lantern slides. Tabloid Rytol is a similar universal de- 

 veloper. If development is frequent it is more economical 

 to make up one's own solutions from chemicals bought 

 in suitable quantities. 



Pyro Soda has the advantage of being among the 

 cheapest as well as the best developers, but it has the 



