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SELECTION AXD USE 



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 resolv- 

 ing power might also have the greatest penetration. Wo confess, 

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

 bined with very high angular aperture, and in this view we be- 

 lieve that we are in accord with the majority of our best micro- 

 scopists. Want ol penetration has been urged against objec- 

 tives of high angular aperture as a serious defect in many in- 

 vestigations in histology and natural history. The advocates 

 of high angles claim that the best way to avoid this difficulty 

 is to use objectives of less magnifying power, and to combine 

 them with high eye-pieces. It is claimed that in this way the 

 same amplification and penetration may be obtained as with 

 low-angled objectives, and with greatly increased distinctness 

 and improved definition. 



Where stereoscopic effect is required by means of the ordin- 

 ary binocular, low-angled objectives give better results than 

 those of higher angle one reason for this being that the- 

 corrections of such objectives are not so easily disturbed as 

 those of the others. But it is a curious fact that low-power 

 objectives of high angle, when used monocularly for examining 

 opaque objects, frequently give an impression of stereoscopic 

 effect which is startling. We feel certain that no one that has 

 his attention once called to this point can ever forget it. 



The late Henry J. Clark, in an article contributed by him to 

 Silliman's Journal, many years ago, suggested another meaning 

 for the term "penetration" in objectives. He referred to the 

 power which objectives of high angle and perfect correction 

 have of pasing through and ignoring, as it were, the various 

 layers of cells or other elements which lie above the special 

 layer in focus, which latter, when seen by a high-angle lens, is 

 clear and distinct, while all the others are invisible. Mr. Stod- 

 der, who has recalled my attention to this article, which I had 



