222 Final Remarks on Immersed Ajpertures. 



figures an an'angement for obtaining full apertures in balsam — the 

 same in principle as that described and carried out by myself more 

 than twenty years ago, and yet " wonders that I cannot see it." 

 He next appears as an advocate of the correctness of Col. Wood- 

 ward's diagram,* wherein he assumes his immersion radiant points 

 close, and yet closer, in order to show that rays of any degree 

 of obliquity can be got through a hemisphere, regardless of the 

 destination of all of them to the focus at the eye-piece, thus carry- 

 ing the argument round again to its commencement. On this 

 Mr. Keith gives his verdict that Col. Woodward is right and I am 

 wrong ; and I may state, in reference to his last communication, 

 that I do not think that it is possible to discriminate and decide in 

 such a complicated optical arrangement as a microscope object-glass, 

 that the spherical aberration is " practically nothing by computa- 

 tion " t and — on paper. It would be a great boon to the makers of 

 object-glasses if this could be done. 



It is still a matter of surprise to me that these gentlemen 

 cannot see or will not admit that if an object-glass, whether made 

 by Mr. Tolles or anyone else, be set at the immersion adjustment, 

 and the angle considered as near Mr. Tolles' 180° as anyone may 

 have the temerity to venture, till it occupies the whole of the front 

 lens (for in dry lenses of lariie aperture the rays nearly fill the 

 hemisphere), and with the focus close to the glass, that on dipping 

 it into balsam, whatever the including aperture may have previously 

 been, the cone immediately falls within 82'^ by the first law of 

 refraction, the back focus remaining the same in both cases. But 

 the bias of the discussion has been to show myself wrong by Mr. 

 Tolles presumably getting some extra immersion rays, if only to 

 the extent of 10°, 5°, yea, even half a degree. For the decision of a 

 scientific fact, glasses besides Mr. Tolles' might be referred to in 

 support, as immersions are made in other countries quite unsur- 

 passed ; but it is a triumph for him only.| 



Finally, a few words concerning the slit in focus of object-glass, 

 for cutting off" all these disputed or false rays. It is difficult to anni- 

 hilate this by theory. Having given the death-blow to Mr. Tolles' 

 extra apertures, it may be treated with but little notice, but cannot 

 be got rid of as a thing of no account or a mere sensational afiair. 

 Col. Woodward says, " This method might be used without giving 

 rise to material inaccuracy when the objective is adjusted for un- 

 covered objects ; but when it is closed to the point of maximum 



* 'M.M.J.,' Nov., 1873, p. 212. 



t Ibid., Sept., 1874, p. 124. 

 ' % " One maker havin": 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." — 

 R. Keith, ' M. M. J./ June, 1874. 



