OBITUARY NOTICE OF JOSEPH JACKSON LISTER 545 



for drawing landscapes ; and I may add that the tripod which he invented for 

 supporting the drawing and the camera is that which is now universally used 

 by photographers. 



In December of the same year occurs an account of an examination of a set 

 of four plano-convex lenses, each consisting of a bi-convex of plate glass and 

 a plano-concave of flint glass cemented to it by varnish, constructed by Chevalier, 

 of Paris. Various interesting observations are here met witli. He found that 

 the maker had done injustice to his own 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 beint: 

 injured by varnish ' (for in Tulley's glasses the constituents of the compound 

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

 that would be derived from cementing, if unobjectionable otherwise, in facili- 

 tating the manufacture. 



He made various trials with these glasses in combination, and remarks : 

 ' I will put down my trials of the glasses as the\' 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 triple object-glasses ' 

 (Tulle3'^'s were triples) ; ' and in successfully combining 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 penetrating power greater than 

 could ever be reached with one alone.' In the light of subsequent events this 

 reads almost like a prophecy. 



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 ^^ is but 33 inch, and 

 Chevalier's combination only -26 inch ; vet 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 discover\- of the two aj>lanatic foci. Each 

 of Chevalier's compound plano-convex lenses when used sini;ly jnesented a bur 

 or coma outwards, but when two of them were combined, this conui. instead of 

 being exaggerated, as might liave been expected, was ' less than with any single 

 glass', while the performance in other respects was satisfactorw 'Observing 



