ACHROMATIC OBJECT-GLASS. 75 



ing on his plan, he produced, in 1827, an achromatic combination 

 of three pairs of lenses, which surpassed anything of the same 

 kind that had been previously executed. From that time, the 

 superiority of the plan of combining three pairs of lenses (Fig. 8 ; 

 1, 2, 3), which should be so adjusted as to correct each other's 

 errors, to the telescopic combinations adopted 

 by Mr. Tulley, may be considered to have been 

 completely established ; and English opticians, 

 working on this method, soon rivalled the best 

 productions of Continental skill. 



14. It was in this country that the next im- I* 

 portant improvements originated; these being 

 the result of the theoretical investigations of 

 Mr. J. J. Lister, 1 which led him to the discovery 

 of certain properties in achromatic combinations, 

 that had not been previously detected. Acting 

 upon the rules which he laid down, practical 

 opticians at once succeeded in producing com- SectionofanAchromatic 

 binations far superior to any which had been 

 previously executed, both in wideness of aperture, flatness of 

 field, and perfectness of correction ; and continued progress has 

 been since made in the same direction, by the like combination 

 of theoretical acumen with manipulative skill. 2 For the subse- 

 quent investigations of Mr. Lister have led him to suggest new 

 combinations, which have been speedily carried into practical 

 execution ; and there is good reason to believe that the limit of 

 perfection has now been nearly reached, since almost everything 

 which seems theoretically possible has been actually accom- 

 plished. The most perfect combinations at present in use for 

 high powers, consist of as many as eight distinct lenses ; namely, 

 in front, a triplet composed of two plano-convex lenses of crown- 

 glass, with a plano-concave of dense flint between them ; next, 

 a doublet, composed of a double-convex of crown, and a double- 

 See his Memoir in the "Philosophical Transactions," for 1829. 



2 The first British Opticians (after Mr. Tulley) who applied themselves to the con- 

 struction of Achromatic object-glasses for microscopes, were Mr. Ross and Mr. Powell. 

 Mr. James Smith did not enter the field until some time afterwards; but, having the 

 advantage of Mr. Lister's special superintendence, he soon equalled, in the lower 

 powers at least, the best productions of his predecessors. With Mr. Ross, his son has 

 been subsequently associated : with Mr. Powell, his brother-in-law, Mr. Lealand ; and 

 with Mr. Smith, Mr. Beck, a nephew of Mr. Lister. These three firms have constantly 

 kept up an honorable rivalry, which has been very advantageous to the perfectionnement 

 of the Microscope; and have maintained a position which is still far in advance of that 

 of all other manufacturing Opticians in this country or the Continent. The lenses pro- 

 duced by each are distinguished by excellencies of their own ; and it would be scarcely 

 possible fairly to assign an absolute preference to either above the others. Among the 

 amateurs who have occupied themselves in the construction of microscopic Achromatics, 

 Mr. Wenham has been the most successful. An American rival has recently been 

 announced, in the person of Mr. Spencer; who, taking advantage of all that had been 

 previously accomplished, is said to have produced combinations not only equalling, but, 

 in some important particulars, surpassing those of English makers. Only one of these, 

 however, has found its way (the author believes) to this country; and not having had 

 the opportunity of seeing it himself, he can only judge of it by report. (See Appendix.) 



