266 CORRESPONDENCE. 



radiant between the objective and the true radiant. No adjustment of 

 the objective can change the relative positions of these two radiants, 

 which are correctly given in the figm"e. Otherwise the criticism seems 

 to me irrelevant, as the only use of the figure was to prove that a ray 

 making a greater angle than 41*^ with the axis could pass into the 

 objective, which seems to be no longer denied. 



The optical principle upon which was based a theoretical objection 

 to Mr. ToUes' objective of 87° immersion angle, is that of total reflexion. 

 That objection being abandoned, the discussion turns upon the course 

 of the rays after entering the objective. It will be found that there 

 is nothing mythical about the back lenses of the instrument in ques- 

 tion, but that they are capable of bringing to a sharp focus rays 

 emanating from an object in balsam at an angle of 87°. 



One maker having made a myth of the " limit," it is probable that 

 the rest will soon follow. But Mr. Tolles richly deserves high praise 

 from all who use microscopes, and all who make them, for perseverance 

 in the mechanical expression of his correct perception of the case, in 

 opposition to high theoretical authority. 



Respectfully, &c., 



R, Keith. 



The Aperture Question. 



To the Edit07' of the 'Monthly Microscopical JoiirnaL^ 



London, May 8, 1874. 

 Sir, — Having heard complaints from several quarters that the aper- 

 ture discussion has become tedious so far as it relates to the point, 

 whether a few degrees beyond the theoretical limit has been obtained 

 or not in an immersion lens upon an object in balsam, the time has 

 come when the question may rest upon its own merits, as the late 

 personalities must appear discreditable by neutral readers who care 

 for the science only. It happens that Mr. Tolles, who is stated to 

 have accomplished the feat, is an optician obtaining his income from the 

 construction of object-glasses, consequently trade feeling has crept in, 

 the acrimony of which might have been avoided had this not been so. 

 The whole evidence against theory and myself has rested on what 

 Mr. Tolles is said to have done. On this the question may "be at an 

 end ; for the first object-glass sent for trial did not show any such 

 result, neither does the last, now in the possession of Mr. Crisp, 

 though specially intended to do so, but proves quite the contrary, 

 even from the evidence of dimensions given by the front lens itself. 

 I submit that I was not bound to be reticent because Mr. Tolles 

 as a maker chooses to engage in controversy. The coming of the first 

 glass was announced by a kind of challenge that it had qualities 

 which " no English glass would be foimd to possess," and curiosity 

 was attracted. I had not the slightest wish to damage the occupation 

 of Mr. Tolles or his agent, and believe that no injury has been done, 

 for Mr. Tolles now probably considers that form obsolete, and not to 

 be repeated. All those that made comparisons of the former glass 



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