ELEMENTARY PHOTO-MICROGRAPHY. 



61 



After proper adjustments, if it be diatoms 

 we are photographing, they will now be seen 

 glittering like pearls upon black velvet. The 

 microscope is connected up with camera, re- 

 focussed, and an exposure made. Longer expo- 

 sure must be given than with direct light. The 

 mirror of the microscope is not required. It 

 is well to make sure that the substage condenser 

 is properly centred with the optical axis of the 

 microscope before commencing, for unless the two 



FIG. 18. 



coincide, the condenser, when sending forth its 

 beam of light, will be like a rifleman aiming at a 

 target but not at the bull's-eye, and consequently 

 the best effect will not be obtained. Moreover, 

 not only must the condenser be correctly centred, 

 but exactly focussed as well, otherwise the rays of 

 light which illuminate the object will not all 

 enter the microscope, but some will diverge in a 

 cone outside of it, and as it is only from the image 

 forming rays passing through the microscope that 



