122 CORRESPONDENCE. 



maximum angle, — but Podura in this case is utterly insignificant, and, 

 this related testing for angle is, is it not ? in the same category. 



As Mr. Wenham, for the second time, refuses to appear again to 

 gainsay what he has distinctly admitted,* — the real issue in my first 

 non-provocative " experiment," - ]" — as against his broad challenge to 

 "anyone,"| *' seem s proper that I should procure further attested 

 measurement here, and, doing so, it shall be without warp of vision or 

 judgment from any theory ; the contrary of which strangely enough 

 Mr. Wenham seems to require ! in his expression alluding to Dr. 

 J. Curtis's statement. Yet unhappily for our critic, all that I have 

 set forth is strictly accordant to accepted theory in all the matter 

 involved, only, Mr. Wenham (it seems) does not understand how ! 

 But perversely it seems to me, and entirely without warrant or need 

 to do so, he pronounces all diagrams wrong unless (as I understand 

 him) drawn for refraction in the front lens according to index of 

 refraction of crown glass, 1*525 (or 1-531), which is a wide error of 

 fact. 



Mr. Wenham is solely responsible for the assumption. I gave no 

 indices of refraction. It was not necessary. I did set forth what was 

 possible, but not limiting myself to use of crown glass only, in the 

 " front." Anyone can conceive of a material in the front having the 

 same refraction in balsam as has common crown glass in air, and 

 what then becomes of the limit of 82° in balsam? This simple 

 question solves the whole matter. I have not suggested it perhaps 

 distinctly before, but, only to leave it to ordinary discernment to 

 discover. 



The quotation, "no collecting power," is not my expression, and 

 Mr. Wenham should not place it to appear so. 



Respectfully yours, 



Eobt. B. Tolles. 



Aperture of Immersed Object-glasses. 



To the Editor of the ' Monthly Microscojncal Journal.' 



Sir, — By your courtesy I have seen a proof of a letter from Mr. 

 Tolles for publication in this number. The optical question of 

 the discussion having ended, I prefer now to make a brief comment 

 on the aperture trial, which however " bald and unsatisfactory " it 

 may appear to Mr. Tolles, I maintain was an accurate and fair one. 

 When the T Vth in question was brought as close as the range 

 of adjustment will allow, apparently it gave a large aperture ; but 

 then the aberrations were such that it would not define objects under 

 any thickness of cover. I set it at a position that gave the best 

 definition on a well-known test, and that angle being measured, the 

 very simple question to be solved was how that same angle became 

 diminished by immersion in water and balsam? In the transfer 

 of course the adjusting collar was not altered. 



* See 'M. M. Jour..' No. xxxvi., p. 292. f 'M. M. Jour.' for July, 1871. 



X 'M. M. Jour.,' No. xxvii., p. 118. 



