136 Transactions of the [S'i?; SJI'tXHI™. 



bi-convex of plate glass and a plano-concave of flint glass cemented 

 to it by varnisli, constructed by Chevalier, of Paris. Various 

 interesting observations are here met with. He found that the 

 maker had done injustice to his ovni instrument by shutting out 

 a needlessly large portion of circumferential rays ; and that when 

 the apertures had been enlarged by increasing the holes in the 

 stops, the glasses performed much better, so as to " give him strong 

 doubts of the figure of these small achromatics being injured by 

 varnish " (for in TuUey's glasses the constituents of the compountl 

 lenses were not cemented together), and he remarks on the great 

 advantage that would be derived from cementing, if unobjectionable 

 otherwise, in facilitating the manufacture. 



He made various trials with these glasses in combination, and 

 remarks : — '*' I will put down my trials of the glasses as they were 

 made. Some of them have surprised me ; and they will show, I 

 think, remarkably, the advantage to science and art of collating 

 the detached labours on the same subject, of distant individuals. 

 The French optician knows nothing of the value of aperture, but 

 he has shown us that fine performance is not confined to tri])Ie 

 object-glasses " (Tulley's were triples) ; " and in successfully com- 

 bining two achromatics he has given an important hint, probably 

 without being himself acquainted with its worth, that I hope will 

 lead to the acquisition of a ]^enetrating pou'er greater than could 

 ever be reached with one alone." In the light of subsequent events 

 this reads almost like a projohecy. 



With respect to a combination of one of Chevalier's glasses with 

 one of Tulley's, he writes : — " The performance of this compound 

 is the finest I have ever seen produced by achromatic glasses, and 

 furnishes, I think, a very important fact. Its virtual focus is 

 •52 inch, while W. Tulley's -/'o- is but '33 inch, and Chevalier's 

 combination only • 26 inch ; yet it goes beyond them both in clear 

 positive power of defining." 



But the most interesting parts of this note are those which record, 

 for the first time, some puzzling appearances in combinations of 

 compound lenses, which ultimately led him to his great discovery 

 of the two aplanatic foci. Each of Chevalier's compound plano- 

 convex lenses when used singly presented a bur or coma outwards, 

 but when two of them were combined, this coma, instead of being 

 exaggerated, as might have been expected, was " less than ivith any 

 single glass" while the performance was in other respects satisfactory. 

 " Observing the advantages resulting from this combination," he 

 " tried some others," among the rest two of Tulley's triple glasses, 

 each of which taken singly was of fine performance. But, instead 

 of unmixed improvement resulting, we find it noted : — " N.B. Each 

 glass se-paraiehj shows a bright object all over the field without 

 bur, and is not far from being achromatic. But, combined, the 



