ELEMENTARY PHOTO-MICROGRAPHY. 57 



his objectives possess power of penetration, defini- 

 tion, flatness of field, and coincidence of visual 

 and actinic foci, he cannot do better than test the 

 value of his lenses by taking a photograph of 

 some well-known object suitable for the power 

 used. Any good book on the microscope will give 

 information on this point. Markings of diatoms, 

 the proboscis of the blowfly, the pygidium of the 

 flea, podura scales, and sections of wood are well- 

 known objects for such a purpose. 



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 

 a loin, tube is the standard, and a negative taken 

 loin, away from the eyepiece will give an enlarge- 

 ment equal to that seen by the eye when looking 

 through the eyepiece. If the camera be now halved 



