THE I'AST AM) F I TV HE OF THE M I Ch'OSCorE 197 



Down to the cmkI of (he lirsl (juarU'r of the iiiiietceiilh 

 century, the simple lens seems to have been the instrument 

 of the investigator. Even as late as 1831, at Robert 

 Brown, the botanist's, advice, Darwin took only a simple 

 microscope on his voyage of research in natural history; 

 and his long work on Cirripedes was in most part con- 

 ducted with the simple microscope and uncorrected single 

 or double lenses. Wollaston's "doublet" came just too 

 late, when the achromatic objective was soon to be com- 

 mon; though, for long, such uncemented "doublets" and 

 "triplets" were made by Zeiss. (Since such combinations 

 are now nearly obsolete, the terms "doublet" and "triplet" 

 are applied, following good examples, in this book, to 

 cemented compound lenses only.) 



The Achromatic Objective. — About 1823, an important 

 event occurred, for Chevalier constructed compound 

 objectives of two or more cemented doublets, each con- 

 sisting of a planoconcave flint and a biconvex crown lens ; 

 he also learned by experience that, to give least spherical 

 aberration, the plane sides must be next the object. The 

 best distances were doubtless determined by trial, not 

 by calculation. Several persons are recorded as having 

 previously tried, for the microscope, achromatic combina- 

 tions, after the pattern of the achromatic telescope objec- 

 tive, invented long before. Chevalier, apparently, still 

 retained the narrow diaphragm of the uncorrected objec- 

 tive, which was removed by Lister, who understood the 

 importance of increase of aperture. Lister, in 1826, 

 caused to be made an achromatic microscope with a 

 compound objective; and in 1830 he published his cele- 

 brated paper on the aplanatic foci of achromatic doublets, 

 which led to the rational manufacture of objectives made 

 of a number of doublets or triplets. The practical con- 

 struction of achromatic objectives did not go farther 

 along the line of correcting each doublet separately; 

 but followed instead the way of making non-separable 

 objectives, in which one compound lens, or more, at the 

 back, corrected errors of the other simple or compound 



