100 PHOTO-MICEOGRAPHY 



only. If a good sound image of the flame edge is not well shown, it is very obvious it 

 is more diflicult to obtain critical light with the same completeness as if it were good ; 

 but condensers must never be expected to give so fine images as objectives, it would 

 make them too costly. To test for definition the condenser may be placed on the 

 microscope, and its performance compared with that of an objective of the same N. A., 

 always remembering that the slide should be turned round the opposite way, i.e., 

 with the cover-glass toivards the mirror and the plane glass of the slip toivards the 

 condenser when fixed on the nose-piece. This, of course, is necessary because the 

 correction has been made, or should have been made, by the optician with respect to 



Fig- 53 



the thickness of the slip, just as he makes the correction for an objective with respect 

 to the thickness of the cover-glass. Another point here comes before our notice : 

 Slips, unfortunately, are of great difference in thickness, some being much thinner 

 than others, just as cover-glasses vary, and it is with pleasure we note that Mr. 

 Conrady has allowed for this variation within a certain limit in liis new condenser 

 of N.A. 0-95 by giving the mount what may be called a correction collar, much after 

 the fashion that dry objectives of high power are made. 



Condensers are manufactured, as before stated, of different N. A., and that of ro 

 is the highest that can be made dry, and all condensers of higher N. A. have to be 

 oiled to the slip with cedar oil, just as the immersion objectives are oiled to the cover- 

 glass. The reason of this is the same as that given in the previous article upon dry 



