222 PHOTOMICROGRAPHY 



all detail clearly, and of the right density to print well. 

 This can be assured, after correct exposure, by carefully 

 watching the plate during development, when a good 

 ruby light is allowable. It is difficult or impossible, 

 however, to watch the course of development of pan- 

 chromatic and, to a less extent, orthochromatic plates, 

 and a mechanical or automatic means for controlling 

 their development has been sought for and found 

 in " Time development." This process is now very 

 largely used, and can be relied upon to give results at 

 least as good as, and generally better than the older 

 methods. 



Time Development. Experiment has shown for a given 

 brand of plates and the same developer used at a definite 

 temperature, and within certain limits of exposure 

 depending on the latitude of the plate, that : 



1. The same degree of contrast or gradation is always 

 obtained by development for a given time. 



2. The density attained in that time depends on the 

 exposure given. 



Most plate makers give the times at 55, 60, and 65 

 F., for which it is necessary to inunerse their plates in a 

 given developer to get a good negative, and similar lists 

 of the times required by well-known plates are given 

 with most proprietary developers. Unless otherwise 

 stated, these times will produce a negative of medium 

 gradation suitable for landscapes, and must be increased 

 by a quarter or a half to obtain the greater contrast 

 desirable in a photomicrograph. Suitable development 

 times are given with the developers in the appendix, but 

 as different brands of plates behave in different ways, 

 they may require lengthening or curtailment to produce 

 the desired result with the particular plate used. One 

 or two trials will determine the correct time in any given 

 case. 



The chief difficulty in time development is to keep the 



