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 



