CORRESPONDENCE. 27 



me pleasure to sliow B. objectives of large aperture with good jiene- 

 tration. Still the fact remains that objectives of very higli aperture, if 

 good, are different things from those of very low aperture ; and when 

 two glasses are to be compared the angular aperture should be taken 

 into consideration. 



It must also be remembered that glasses of great aperture, if good, 

 are more costly than those of small aperture ; because the proper cor- 

 rections for sjihericity and colour involve, and require, more skill, and 

 a better knowledge of pure optics in the optician, and also more time 

 and more care in the mei*ely mechanical work ; so that the cost of 

 a " one-quarter " of 170° is nearly or quite twice that of the same 

 focus of 100°, of equal quality in all other respects. 



Charles Stodder. 



Eeply to Mr. Stodder, " B," and IMr! Edwin Bicknell. 



To the Editor of the ' Monfhli/ Microscopical Journal.^ 



Washington, D.C, November 21, 1871. 



Mr. Editor, — Tliere is nothing, I think, in the argumentative 

 parts of Mr. Stodder's letter of July 20th * which calls for any reply 

 from me, but the paragraph on page 203 concluding " all these facts 

 I presume Dr. W. will in due time communicate to the public," 

 demands a few words. 



In January last I ordered, through Mr. Stodder, a high-power 

 immersion objective of Mr. Tolles for the Army Medical Museum, 

 " to be constructed with special view to the study of lined test- 

 objects," leaving details of precise power, angle, price, &c., entirely to 

 the maker, and stating to Mr. Stodder that it was my desire " to have 

 in the Museum an objective which you will be willing to have me 

 exhibit as a selected specimen of Mr. Tolles's very best work." 



In the latter part of June I had the pleasure of a visit from Mr. 

 Tolles, who brought with him the new immersion 1th referred to in 

 such complimentary terms in my Note on the resolution of Amjjlii- 

 pleura pellucida.'\ Mr. Tolles had with him at the same time two 

 immersion yVths and an immersion yV^^' ^^® performance of which 

 was exceedingly creditable ; but as it was essentially similar to that 

 of other objectives of his previously referred to by me in this Journal, 

 I limited my published remarks to the one of which I could con- 

 scientiously speak as superior in definition to any similar power which 

 I have examined. 



At the same time I engaged Mr. Tolles to undertake the con- 

 struction of a higher power objective for the Museum, which, however, 

 has not yet come to hand. Should it fulfil Mr. Tolles's expectations 

 it will give me as much sincere pleasure as it can to him or any of 

 his friends, and I will promptly make the facts public. 



My September paper also contained my determinations of the true 



* This Journal, October, 1871, p. 201. f IWd, Sept., 1871, p. 150. 



