90 



THE AMEEICAN MONTHLY 



[May, 



almost any microtome ; and is a 

 very simple piece of apparatus to 

 manage, and gives results, such as 

 I have never seen attained by any 

 one using other methods. 



Second. — The next requirement 

 of the student, if he really desires 

 to attain the acme of success in 

 the demonstration of histological 

 truths, is some means, after obtain- 

 ing good preparations to thoroughly 

 examine his work. In order to do 

 this satisfactorily to himself, he 

 must be in possession of means to 

 obtain various degrees of amplifica- 

 tion, and often of the very highest. 

 In the very recent past, it has been 

 thought possible, by high author- 

 ities, to accomplish this end, only 

 by resorting to high power objectives 

 of ^ or ^3-incli focus, and even 

 higher. 



The best opticians are ever on 

 the alert, to take advantage of even 

 slight chances for improvement 

 and variation in former methods, 

 to aid the worker with that instru- 

 ment, which has unraveled so many 

 mysteries. And it seems hazardous 

 to predict, even now, with such 

 perfect instruments as we have, to 

 what degree of perfection such 

 opticians will yet carry moderately 

 high powers, tliereby enabling 

 the microscopist of slender purse, 

 who could never for a moment 

 think of buying a ^inch lens, to 

 get an objective which will do away 

 with all need of, or desire for such 

 necessarily expensive lenses, and at 

 the same time furnish him with 

 a lens which will afford him far 

 more satisfaction, and better results 

 than are possible with ordinary 

 lenses of great power. I say ordi- 

 nary, for the reason that all objec- 

 tives of extremely high power, ow- 

 ing to their difficult construction, 

 are ordinary as compared with the 

 moderately high powers of the best 



makers, whicl 



may 



be said in a 



very few cases to be extraordinary. 



And this brings me to write of 

 what was the object of this 

 article, viz. : to call the atten- 

 tion of young, earnest seekers after 

 histological as well as biological 

 knowledge, to such lenses as I have 

 found to far excell all others I have 

 ever seen, for this class of work. 



Much has been said and written 

 about the value of objectives hav- 

 ing high interior angles, say 90° and 

 upwards, for histological study. 



But most of what has been said, 

 has been inserted in a secluded part 

 of an extended article, calling at- 

 tention to the results obtained by 

 the use of these lenses on " lined 

 test-objects ; such as Amphiplura 

 pellucida and other diatoms. 



So that I find the impression 

 is strongly prevalent among micros- 

 copists who care but little for this 

 "trivial use of the instrument," 

 that "those high-priced and high- 

 angled objectives are of no use, ex- 

 cept to amuse a few Diatomists." 

 But such is, I believe, by no means 

 the true state of the case. 



Even a water-immersion objec- 

 tive, having 105° interior-angle, 

 will so far excell another of only 

 120° or 140° air-angle ; that the 

 image of the latter will be unsatis- 

 factory to any careful observer who 

 delights in having the minute de- 

 tails of the object brought out with 

 sharply defined outlines. When 

 we institute a comparison between 

 the glycerin, or more especially the 

 homogeneous-immersion objectives, 

 such for example as the latest pro- 

 duction of Mr. K. B. Tolles of 

 Boston, and the water-immersion 

 of 105°, a very decided improve- 

 ment is noticed in favor of the 

 homogeneous-immersion ; owing, as 

 I believe, in part to the great in- 

 crease of angle that is attainable 

 with the latter. For, when the 

 immersion medium is water, this 



