PRACTICAL PHOTO-MICROGRAPHY. 67 



great delicacy of detail or color the real difficulties of photo- 

 micrography we require a plate thickly coated with an emul- 

 sion containing a handsome proportion of silver haloid. More- 

 over, with the first class of subject, over-exposure is less to be 

 feared than the reverse, so that rapidity of emulsion is rather 

 to be desired than avoided ; while with delicate subjects the 

 exposure, even with high powers, is never prolonged to incon- 

 venience, except in very exceptional cases where oblique light 

 is used. A good " portrait " plate, then, is recommended for 

 the ordinary run of low power work, while for the higher 

 nights of " critical images," bacteria and the like, a plate 

 should be thickly coated, not too rapid, and capable of giving 

 a plucky, or even a "hard," negative at will. 



The two great factors in the late advances in photo-microg- 

 raphy have been : 1st. The introduction of rapid emulsion ; 

 2d. " Color correct," or " orthochromatic " photography. (We 

 omit for the present a third factor, which is optical in its 

 nature). It may be asserted that the man who wishes to 

 produce photo-micrographs of general utility, and still more, 

 he who aspires to march anywhere near the van of the 

 photo- micrographic army must master orthochromatic pho- 

 tography. There is no getting round this fact. The majority 

 of the most useful objects are only to be rendered to the best 

 advantage by color-correct plates, and a large number of ob- 

 jects can not be photographically rendered at all without such 

 plates ; and there are objects which will not be photographed 

 until orthochromatic photography is perfected. The beginner 

 should, therefore, provide himself with some orthochromatic 

 plates for a start ; as he becomes accustomed to their manipu- 

 lation he is advised to orthochromatise his plates for himself, 

 if he has facilities for drying plates. The subject of ortho- 

 chromatics, though far too wide, as a whole, for full treatment 

 in this book, will be treated as carefully and as fully as the 

 author is able -to treat it in a single chapter. 



Under the heading of lantern-slides we shall give a 

 description of the wet collodion process, which must suffice for 

 the reader's present needs ; the process given in that chapter 

 will serve to produce negatives as well as positives. More 



