LISTER'S DISCOVERY 



355 



is of interest. He found them much stopped down, and in one 

 instance he opened the stop and improved the effect. Lister says : 

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

 he has shown us that fine performance is not confined to triple 

 objectives ; 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 pene- 

 trating l power greater than, could ever be reached with one alone.' 



At this time Professor Amici, of Modena, one of the leading 

 minds who assisted in giving its form to the modern microscope, had 

 been baffled by the difficulties presented by the 

 problem of achromatism, and had laid it aside 

 in favour of the reflecting microscope, but he 

 now returned to the practical reconsideration of 

 the production of an achromatic lens. As a 

 result he appears to have constructed objectives 

 of greater aperture than those of Chevalier. 

 He visited London in 1844, and brought with 

 him a horizontal microscope, the object-glass 

 being composed of three doublets, which pro- 

 duced a most favourable impression. 



Meantime, in this country, Mr. Lister 

 Ill-ought about an important epoch in the evo- 

 lution of the achromatic object-glass by the dis- 

 covery of the two aplanatic foci of a combination. 

 It had occupied his mind for several years, but 

 in January 1830 a very important paper was 

 read to, and published by, the Royal Society, 

 written by him, in which he points out how the 

 aberrations of one doublet may be neutralised 

 by a second. 



As the basis of a microscope objective, he 

 considers it eminently desirable that the flint 

 lens shall be plano-concave, and that it shall be 

 joined by a permanent cement to the convex 

 lens. 



For an achromatic object-glass so constructed 

 he made the general inference that it will have 

 on one side of it two foci in its axis, for the 

 rays proceeding from which the spherical aber- 

 ration will be truly corrected at a moderate 

 aperture ; that for the space between these two 

 points its spherical aberration will be over-corrected, and beyond 

 them, either way, under-corrected. 



Thus, let a,'b, fig. 309, represent such an object-glass, and be 

 roughly considered "as a plano-convex lens, with a curve, a c b, 

 running through it, at which the spherical and chromatic errors 

 are corrected which are generated at the two outer surfaces, and 

 let the glass be thus free from aberration for rays,./', d, e, g, issuing 



1 ' Penetrating ' meant ' resolving ' power in those clays ; he alludes, therefore, to 



increase of aperture. 



A A 2 



FKJ. :;<>!>. The two 

 aplanatic foci of an 

 optical combination. 



