192 HOW TO SEE WITH THE MICROSCOPE. 



pal advantage in the use of the " reflex" with sunlight 

 is in arriving at a knowledge of surface markings, and 

 for this purpose it is indeed very valuable. Thus work- 

 ing the " reflex " by sunlight, the mirror must be manip- 

 ulated so as to produce the same effects as have been 

 described by moving the hand-lamp and conversely. 

 The mirror may be substituted for the hand-lamp when 

 working in the evening, but the most favorable results- 

 are obtained with the light direct. This reflex and sun- 

 light illumination is especially desirable when one wishes 

 to trace out structure situated in one particular plane, 

 to the exclusion of that lying in adjacent planes. In 

 the general squabble to produce the so-called penetra- 

 tion, this very important item has been lost sight of. 



We are now ready to consider a matter which has 

 been alluded to on a preceding page. It has been 

 already stated that the maximum performance of ad- 

 justable objectives can only be secured when such object- 

 glasses are worked at the point of their maximum aper- 

 ture, and that this point is by no means a fixture but 

 varies with different objectives. Every observer should 

 then ascertain for himself as to the proper handling of 

 his object-glasses in this particular. Methods will now 

 be given which, although but approximate, are suffi- 

 ciently precise for the use of the practical manipulator. 



For the purpose of testing the point of maximum 

 aperture for object-glasses having apertures, say from 

 40 to 175, proceed thus: Place the objective in posi- 

 tion on any good stand having a thin stage and mirror 

 attached to radial arm. Commence by focussing any 



