PRACTICAL PHOTO-MICROGRAPHY. 27 



Doubtless, any well made stand may be used with complete 

 success. Much very fine work has been produced with cheap 

 microscopes not designed for photo-micrography ; so that any 

 one possessing any good microscope and a moderate amount of 

 ingenuity and neatness of hand need not despair of success, 

 should his pocket not admit of the purchase of a new and 

 specially adapted stand. One of our diagrams will show how 

 easily any stand may be used for photo-micrography at an odd 

 time or in a casual way. 



The Objective^ or Object Glass is, on the whole, the most 

 important part of the apparatus. The first essential is that 

 it be u corrected for photography."* Objectives made for 

 ordinary purposes of observation are usually " over-corrected," 

 a term that requires explanation. The rays which give the 

 best visual effect being less refrangible than those exercising 

 the greatest influence on chemical compounds such as we 

 employ in photography, the latter rays come to focus behind 

 the focal point of the visual rays, and naturally lenses for 

 ocular observation only are corrected for visual and not for 

 actinic rays. Moreover, in order to overcome certain im- 

 perfections of image that would arise from the interposition 

 between the object and the lens of the glass disc usually 

 covering the object, and also to meet the fact that ordinary 

 eye -pieces are not made achromatic, the objectives are still 

 further corrected in such a way as still further to separate 

 the visual from the actinic focus. The greater the principal 

 focus (i. e. the lower the power) of the lens, the greater the 

 distance between the foci, so that while with a high 

 power objective the foci may practically correspond, with 

 a low power they are so sensibly separated as to produce a 

 blurred image in photography while visually the image was 

 quite sharp. Of course, this would be fatal to photo-micro- 

 graphy. We must, therefore, either procure objectives cor- 

 rected for photography, which is the best plan by far, or 

 we must by the use of a supplementary lens so alter the 

 whole combination that the visual and chemical rays shall 



*The "correction," if not in the objective itself, may be obtained by 

 use of an ocular made for the purpose, Corrections for chromatic and 

 spherical aberations are, of course, essential to all objectives. 



