124 Transactions of the V^^^'uXn^rc^w 



refer to Dr. Eoyston-Pigott's paper " On High Power Definition," 

 and to my own paper " On the Equilateral Prism." Considering 

 a perfect microscope as consisting of two parts, a magnifying 

 apparatus and an illuminating apparatus, Dr. Pigott proposes to 

 weed out, as it were, an effective portion of the small residuary 

 spherical aberration of the best objectives, and I have ventured to 

 propose a principle of illumination which has not hitherto been 

 avowedly advocated. When Dr. Wollaston recommended for an 

 illuminating lens one of three-fourths of an inch in focal length, in 

 which the microscopic object was placed in a vortex of foci, where 

 the rays crossed in a thousand points both before and after they 

 fell upon the object, he failed to realize the true method of illumina- 

 tion. Spherical and chromatic aberration are equally injurious in 

 either of the essential parts of a microscope. The equilateral 

 prism used as a condenser and the hemispherical lens used as a 

 prism, are free from both these errors. They supply, the one 

 a condensed, the other a simple, single beam of parallel light ; and 

 a microscopic object, under such illumination, has virtually the 

 advantage of being illuminated as by the sun. Natural light and 

 shade are secured, and objects, like the valves of the Diatomaceje, 

 which hitherto have been shrouded in a haze of interpretations, are 

 truthfully presented to the observer. I therefore look upon the 

 prism as a kind of '^ Zajiluiath-iManeah" a revealer of secrets. 



Dr. Pigott's work is more arduous, and it certainly met in the first 

 instance with an encouraging amount of opposition, — encouraging, 

 I mean, to one who knew — and was prepared to defend — the right. 

 For my own part, I must honestly say, and I am not alone in my 

 opinion, that I did not believe a word of it. In point of fact, I was 

 sure that Dr. Pigott's " beaded scale " was not the true test scale. 

 True, we were all taken by surprise and uttered our criticisms 

 freely, yet, in my own defence, I must be allowed to say that I 

 do not concur in the personal comments reported in our ' Proceed- 

 ings ' and now willingly cancelled. I felt, however, that I had sat 

 too long at the feet of my old friend, Andrew Eoss, to admit the 

 possibility of his good work being vitiated by this newly-announced 

 error, and what I did not look for I did not find, — but only because 

 I did not look for it, not because it did not exist. There is un- 

 doubtedly in our best objectives a residuary spherical aberration, — 

 small, I admit, — but it is unnecessary to say to a microscopist that 

 its injurious efi'ect upon the magnified image of an object varies 

 directly as the square of the power of the eye-piece. It would be 

 beyond the scope of a President's Address to point out how this 

 small but injurious amount of error may be detected and dimi- 

 nished. This must remain as a fundamental problem in the 

 Opticians Euclid. It is enough to record the fact of improve- 

 ment as one of the salient points of the year. I will therefore 



