

ELEMENTARY PHOTO-MICROGRAPHY. 21 



its use is more than compensated for by the com- 

 pactness of the arrangement, whereby the operator 

 may see or control the whole of his focussing and 

 lighting from one position. A trifle longer ex- 

 posure is of no moment to a beginner, since the 

 time lost in this way is saved in the facilities 

 offered for focussing. The errors of objectives also 

 are corrected by eyepieces, and good results can 

 be obtained when using them for photographic as 

 for visual purposes. The C eyepiece, although 

 giving great power of enlargement, is not recom- 

 mended when a low power will do. Yet the writer 

 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 



