ELEMENTARY PHOTO-MICROGRAPHY. 51 



If no suitable condenser be at hand, a bull's- 

 eye may be requisitioned and made to do good 

 service, especially for large objects. Paste on it a 

 piece of black paper to block out the central rays, 

 and use it in place of a condenser (fig. 18). A 

 spot lens or a Wenham parabolic reflector to fit the 

 substage will also give a dark ground with the 

 object illuminated thereon. 



FIG. i 8. 



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, refocussed, and an exposure made. Longer 

 exposure 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 

 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 



