78 THE A B C OF PHOTO - MICROGRAPHY 



But I can do no more than mention the pos- ; 

 sibility of using high powers with opaque ob- 

 jects at this time and place. We are dealing 

 only with the ordinary cases of every-day 

 work. Such are the examples shown in Figs. 

 15 and 1 6. The former is from a strewn slide 

 of diatoms, mounted dry on a dead-black back- 

 ground, especially for examination by reflected 

 light, and it certainly is a very beautiful object 

 under the microscope a beauty well repeated 

 in the photograph. Attention is directed to a 

 curious optical effect presented by this pic- 

 ture. Viewed with the page in ordinary posi- 

 tion, many of the discs look like shallow sau- 

 cers containing other diatoms of smaller sizes. 

 Reverse the page and they all appear convex, 

 with the smaller forms pressed into their bulg- 

 ing surfaces. This is caused by the illuminat- 

 ing pencil of light being projected obliquely 

 across, thereby casting slight shadows of their 

 delicate margins. Fig. 16, having been made 

 with diffused daylight falling direct upon the 

 object, is nearly free from shadows, and shows 

 no change when thus reversed. The two 

 negatives were made under the following con- 

 ditions. Fig. 15: Objective, student's f; 

 ocular, aplanatic i inch ; radiant, acetylene, the 

 rays thrown obliquely across surface of ob- 



