36 ELEMENTARY PHOTO-MICROGRAPHY. 



good book on the microscope will give information 

 on this point. Markings of diatoms, the proboscis 

 of the blow fly, the pygidium of the flea, podura 

 scales, and sections of wood are well known objects 

 for such a purpose. 



To measure the amplification with any extension 

 of camera after photographing the object, a micro- 

 meter is placed on the stage, and the divisions on it 

 are photographed, keeping the camera extension 

 unaltered. 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 ten inches away from the eyepiece 

 will give an enlargement equal to that seen by the 

 eye when looking through the eyepiece. 



If the camera be now halved in length the mag- 

 nification is halved, and if the camera be doubled 

 the increase is in proportion. 



A table showing the enlargement due to the 

 various eyepieces and objectives is given by most 

 makers in their catalogues, and may be referred to 

 for ready reference. 



Crystals are too transparent for ordinary photo- 

 graphy, but can be brought out by the aid of the 

 polariscope, which shows their form in many 



