392 HOW TO SEE WITH THE MICROSCOPE. 



These two objectives were constructed expressly for me, and 

 were especially intended for conjunct use they are in truth, 

 companion glasses ; either of them will display the 19th band 

 by oblique use of artificial light, and also the markings of An- 

 gulata by central lamp illumination. Of the two, the first- 

 named has the greatest working distance, and, for reasons 

 already given, will work at maximum performance over covers 

 l-50th of an inch in thickness, and in the hands of one thor- 

 oughly conversant with its functions, it is, perhaps, the most 

 generally useful glass. 



I have here still another objective of plus 180 of aperture. 

 This glass was intended to be a compromise, and to serve to a 

 great degree the purposes both of the l-6th and the l-10th before 

 mentioned. The glass I now refer to has at " closed " a balsam 

 angle of 97 ; at open point, 92. The compromise is thus appar- 

 ent : this l-6th was made to my order for the gentleman who tills 

 the executive chair of this society. It asks no favors of M. 

 Nobert, nor of a centrally posed Angulata, and when one is con* 

 fined to al-6th alone, a glass of this construction will prove of 

 great value. 



These conditions, which have still greater force when we 

 include the work of various opticians, need attention and study, 

 in default of which you may expect diffraction lines and dif- 

 fraction borders; and, perchance, our old friend, the " Nas- 

 myth," may put in an appearance I 



I repeat, that it was the dominant purpose of these desultory 

 remarks to call your attention to the importance of microscope 

 instrumentation, and to the manipulations connected therewith. 

 To the fact that it is a study, a profession in itself, and an 

 accomplishment that must be fully mastered at the outset, 

 before one can investigate with precision, or have claim to the 

 confidence of others. In point of fact, the most expert manip- 

 ulator will constantly have something to learn there is no 

 landing place where one can stop and rest. The world pro- 

 gresses, and so should the microscopist pari passu. 



I take it for granted that nothing I have said will lead you to 

 suppose that I am opposed to the legitimate use of low-angled 

 objectives; nothing could be further from my purpose. When 



