CHAPTEE XIII. 

 EXPOSURE. 



THERE is not a subject more important, nor any more diffi- 

 cult to deal with, than this one. "We shall make it even more 

 important iu photo-micrography than it is in general photog- 

 raphy, because we propose to advise the reader not to alter to 

 any serious extent the constituents or proportions of his devel- 

 oping solution. It is a more difficult subject here than even 

 in general photography, for whatever be the colors of the ob- 

 jects actually photographed in the latter branch, there is al- 

 ways a very large amount of reflected white light which to a 

 vast extent lessens the difficulties arising from the colors of 

 the objects themselves. Had we to photograph a landscape 

 entirely by transmitted light, were such a thing possible, the 

 result would be curious ; in photo-micrography practically 99 

 per cent, of our light is transmitted, opaque objects being 

 rarely photographed. 



We may state at once that we do not approve of the sys- 

 tem of trying to give rules for exposure ; rules and tables 

 doubtless assist the beginner at first but leave him helpless in 

 the end. Moreover, it is futile to attempt to give rules, for it 

 is impossible to take into account the most puzzling of all 

 photo-micrographic conditions , that of color. If we worked 

 only on colorless objects, we could easily give a most useful 

 code of exposures, but no such state of thing obtains, a color- 

 less object is very rare in photo -micrography. We propose 

 rather to begin at the other end, and to inform our reader 

 how to know after exposure where he has erred ; and by this 

 means he will not only very soon arrive at the proper exposure 

 for the particular object in hand, but he will gain experience 

 by every exposure he makes. By far the nearest approach to 

 a scientific judgment of exposures that we know is the table 



