24 ELEMENTARY PHOTO-MICROGRAPHY. 



has been able to get capital results with C eyepiece 

 when the same magnification had been obtained by 

 a low eyepiece and a high objective with less success. 



The magnification of the eyepiece remains the 

 same, irrespective of tube length, but the pene- 

 trating power of the objective, or depth of focus, 

 decreases with the increase of both numerical aper- 

 ture and magnifying power, inversely as the N.A. 

 and also inversely as the square of the magnifying 

 power ; from which it will be seen that a low 

 power, such as a 2in., will focus and photograph 

 all the different planes of a thick specimen more 

 clearly than a higher power would do. Therefore, 

 when only good outline and principal points of a 

 coarse object are needed, use always the lowest 

 power sufficient to give the desired details ; but 

 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 



