COERESPONDENCE. 99 



That the limiting augle at which rays could be admitted into balsam 

 through a flat plate of glass imposes any natural limit to the angle up 

 to which an object-glass could collect image-forming rays, supposing 

 them to have got into the balsam, is absurd. 



In the discussion on the immersion aperture question, the point 

 insisted on by Dr. Woodward, Professor Keith, and Mr. ToUes, is, that, 

 by means of the immersion principle, image-forming rays beyond the 

 angle 82'^ from a balsam-mounted object — or from any object under 

 equivalent conditions— can be transmitted into the optical image: the 

 question then refers only to rays from balsam beyond the augle of 82"^. 

 Dr. Woodward's demonstrations, supported by Professor Keith's com- 

 putation, were brought forward in illustration of the fact that by a 

 properly devised objective such rays are transmissible. As Mr. Wen- 

 ham rejects Professor Keith's diagram, he is bound to prove it erroneous, 

 by a graphical method if he likes, marking in the same data. It will 

 not do fur him to reject the diagram on the ground that the data were 

 furnished by Mr. Tolles to " suit the proposition ;" for, since this dis- 

 cussion has been revived, M. Prazmowski, of the firm of Hartnack et 

 Prazinowski, the well-known oj)ticians of Paris and Potsdam, has sub- 

 mitted for my inspection a number of his computations referring to 

 immersion lenses made any time during the last fom-teen years, every 

 one of which contains distinct tracing, by the trigonometrical method, 

 of rays from the conjugate focus of greater angle than those trans- 

 missible by a dry objective. It is therefore evident that formulae, 

 involving the transmission of an angle of image-forming rays greater 

 than corresjionds to the maximum air-angle, have been known during 

 this number of years. 



Mr. Wenham's demonstrations seem to have been based on the 

 assumption that his diagram in the 'M. M. J.,' No. xxv., p. 23, ex- 

 plained the 'whole theory of the action of the immersion system. 

 But the rays there figured are merely the rays corresj)ondiug to those 

 transmitted by the diy combination, — which are not concerned in the 

 matter disputed. And in his so-called demonstration in No. xlvii., 

 p. 231 et seq., the image-pencil belonging to the air-limit theory is the 

 only one with which he deals, — he takes fur granted that the flat jjlate 

 limit of refraction from balsam or glass into air — 82° — is the natural 

 limit beyond which no object-glass can collect image-forming rays. 

 Such an objective as the one to which he referred cannot be said to 

 have an immersion aperture in the proper acceptation of the term; 

 because, if its construction be such that its maximum ajierture is ex- 

 pressed by the air-pencil of 170" and no more, when water is intro- 

 duced between the objective front and the cover-glass, the lens will 

 no longer render " the image of a point as, approximately at least, a 

 point," — it admits of no adjustment by which a true immersion focus 

 can be obtained, consequently the immersion aperture cannot be deter- 

 mined. That demonstration does not really touch the " balsam aper- 

 ture question." 



As Mr. Wenham still insists * that the angle 82"^ from balsam is 

 the natural limit, or fidl aperture, "beyond which no object-glass can 



* 'M. M.J.,'No. Ixxxv., p. 48. 



