SUPPLEMENT. 381 



ported by Dr. Beale, and generally endorsed by microscopists 

 of Europe and America. 



I have not the time this evening, nor would I weary your 

 patience by the discussion of this dogma of penetration, any 

 further than to admit that it was true ten years ago, when first 

 announced by Dr. Carpenter, but can have no possible present 

 force in reference to the wide aperture glasses of to-day. I am 

 also quite prepared to grant that a spectacle lens of sixty inches 

 focus needs less skill in management than a duplex l-10th of 

 plus 180 of aperture ! 



After all, I get considerable comfort out of the quotation 

 already presented. Dr. Beale says, " that glasses with a high 

 angle of aperture admit much light, and define many structures 

 of an exceedingly delicate nature, which look confused when 

 examined by ordinary powers." Notice the wording ; he does 

 not say diatoms. He does say, u many structures of an exceed- 

 ingly delicate nature, which look confused when examined by 

 ordinary powers." It is a candid and manly admission, and I 

 honor his candor. It is the truth, the whole truth, and nothing 

 but the truth ; while for brevity of form, or clearness of expres- 

 sion, these half dozen words leave nothing to be desired beyond 

 what is here so forcibly stated ; and I beg of you not to lose 

 sight of the fact that if this was true of wide- angled glasses 

 made in 1865, it is equally so of those of 1876. 



Another fact will bear to be borne in mind. It is this : when 

 Dr. Beale wrote in 1865, he was very far from being expert in 

 the use of wide apertures. I will read to you further from the 

 same paragraph: "In order to adjust the object-glass, it is 

 first arranged for an uncovered object ; then any object covered 

 with thin glass is brought into focus by moving the body of the 

 microscope ; next, the ring which carries the third lens is 

 screwed round until any particles of dust upon the upper sur- 

 face of the glass are brought into. focus. The glass is* then cor- 

 rected for examining the covered object which may be brought 

 into focus." It will be seen that the method of adjustment 

 adopted by Dr. Beale is precisely the same as that used by 

 many for measuring the thickness of covering glass, and when 

 the milled head of the fine adjustment is properly graduated, and 



