22 ELEMENTARY PHOTO-MICROGRAPHY. 



for resolving the structure of any particular plane 

 in the specimen and getting all detail possible, 

 then use an objective with a large N.A. Of two 

 objectives having the same power but different in 

 numerical aperture, the one with the largest N.A. 

 will show the greatest detail. 



Remember that clearness of definition must 

 never be sacrificed for the sake of a larger picture. 

 A crisp negative may be enlarged, whilst a larger 

 image wanting in sharpness is valueless. 



For dark ground illumination and for polari- 

 scope, the eyepiece may be dispensed with at the 

 sacrifice of magnification. Great care, however, 

 must be exercised when not using the eyepiece, or 

 the reflection from the interior of the body tube 

 will manifest itself in the form of a bright central 

 spot on the negative. 



The use of an eyepiece removes this entirely, 

 but by way of object lesson let the reader take a 

 microscope that is not dead blacked inside the 

 tube, and throw the image of an object on to a 

 sheet of paper. After getting even illumination 

 with the eyepiece in position, remove the eyepiece, 

 and immediately a bright central spot of light will 

 appear on the paper. If the paper be now moved 

 to or from the end of the microscope, the bright 

 spot of light will gradually expand into a bright 

 ring of light, and then resolve itself into a point 

 again. The whole surface of the illuminated disc 



