106 HOW TO SEE WITH THE MICROSCOPE. 



at all, and is, in fact, but a medium power glass the 

 general verdict being that it is an indifferent one at the 

 best. 



Fifth. That the capacity of an objective to display 

 markings on balsamed and difficult tests, at or near the 

 limits of its aperture, is no index of angular aperture. 

 In the case presented, if it so be that the eighth, when 

 worked at 140, gave fair and distinct images of the sur- 

 riella, then there is no reason to dispute the angle a& 

 claimed for the glass ; let the experiment be repeated 

 an using angle of 120, and over dry as well as bal- 

 samed mounts. 



The above may be taken as representing individual 

 differences of opinion. Either of the conclusions pre- 

 sented have been urged on our attention time and time 

 again. To the second, third, and fourth, the author gives 

 his assent, and has never allowed an opportunity to pass 

 without exposing by actual demonstration the fallacy 

 contained in the fifth. 



From the preceding, then, it becomes apparent that 

 there is much difference of opinion as to what consti- 

 tutes angular aperture or the measurement thereof. 

 Until there can be some more precise plan arrived at, 

 let the purchaser of any objective imperatively demand : 



First. That the objective, in general, work with its 

 full vim fully up to the* limits of the aperture claimed 

 for it; that the images be strong, vigorous, brilliant, 

 and without distortion, and that such images shall not 

 be surpassed in any particular by any similar glass, 

 without reference to its angle. If it be that you have 



