ELEMENTARY PHOTO-MICROGRAPHY. 71 



seopist, for undoubtedly it was the attempt to make 

 visible all the minute markings of the diatom that 

 brought about such a great improvement in 

 microscopic objectives. 



In fact, only objectives of large aperture and 

 first-class construction in the hands of a skilful 

 operator will reveal the dots, beads, and lines of 

 certain test diatoms which run up to 100,000 per 

 inch. Plate IX. shows a pretty effect obtained 

 by grouping diatoms along with the " crown and 

 anchor." For one in search of a hobby, the col- 

 lection and examination of diatoms will speedily 

 yield a delightful pastime. 



To measure the amplification of the object 

 photographed with any extension of camera, a 

 micrometer is placed on the stage, and the divi- 

 sions on it are also photographed, keeping the 

 camera extension unaltered. Or they may be 

 marked with pencil on the ground-glass screen and 

 distances taken with compasses. The enlargement 

 is measured, and divided by the known distance 

 between the spaces on the micrometer. Thus if 

 i-iooin. on the micrometer measures lin. on the 

 photograph, the enlargement is 100 times. Or the 

 same result may be got by means of the camera 

 lucida, if the paper be placed as much below the 

 camera lucida prism as the ground-glass screen was 

 from the eyepiece. For purposes of measurement 



