52 SELECTION AND T7SE 



attention called to this subject, and after careful study 

 he concluded that if he could only procure a durable 

 glass of greater refracting power than that ordinarily at- 

 tainable, the angular aperture might be greatly increased. 

 He at once went to work, and after many experiments, he suc- 

 ceeded in producing a glass which enabled him to attain imme- 

 diately an angular aperture of 146. As early as 1857 he had 

 produced a l-12th with an angular aperture of 178. His objec- 

 tives had corresponding excellence in other directions, and 

 from that time forward this country has been noted for the 

 excellence of its objectives, and especially for their great resolv- 

 ing power. We may note in passing that glass of great re- 

 fractive power, combined with sufficient -hardness and dura- 

 bility, is now produced as a regular article of commerce. 



Penetrating Power. As previously stated, penetrating 

 power, in the early days of microscopy, meant precisely what is 

 now understood by resolving power. Now, however, penetrating 

 power means the extent to which an object-glass shows the depth 

 or thickness of an object. This is a very important feature for 

 some purposes, particularly in histological work, as by it we are 

 enabled to discover the relations between the different parts. 



Until within a few years it has been accepted, as a thoroughly 

 demonstrated fact, that penetrating and resolving power always, 

 of necessity, exist in inverse ratio to each other, for it is always 

 found that, other things being equal, resolving power increases 

 with the angle of aperture, while penetrating power decreases. 

 Of late, however, it has been claimed that certain lenses of great 

 resolving power and high angle possess great penetrating power. 



In attempting to reach a sound conclusion on this point, it 

 must be borne in mind that resolving power does not depend 

 wholly upon angular aperture. Two objectives of the same 

 angle may have very different powers of resolution, on account of 

 the degree of perfection to which the corrections have been 

 carried, and it is quite possible that a lens of great resolving 

 power may have a lower angle than another objective which it 

 excels in this respect. In this case the lens of greatest resolving 

 power might also have the greatest penetration. We confess, 

 however, that we cannot see how great penetration can be com- 



