502 



G. C. KAROP ON POCKET MAGNIFIERS. 



appear to have been abandoned. It was to Wollaston, again, in 

 1829, that opticians were indebted for a doublet, which, simple as 

 it was in conception (merely a Huygenian ocular reversed), yet, 

 on account of its components being adjustable to one another, 

 effected a sensible correction of the aberration. But the relative 

 distance apart of the lenses and their respective foci as fixed 

 by Wollaston — viz., focus of lower lens to upper as 1 : 3, distance 

 of plane surfaces 1*4 to 1-5 of the focus of smaller lens— were 

 found by Pritchard to be capable of variation with advantage, 

 and he also introduced a diaphragm between them, which still 

 more improved their performance. Chevalier went a step further. 

 His doublets, like Wollaston's, consist of two plano-convexes, but 

 of equal foci, though of unequal diameter, the one next the eye 

 being much the smaller. This allows the lenses to be brought 



Adams. 



Wollaston (1829). 



Chevalier. 



close together, and to pass more light ; moreover, the working 

 distance is also increased. If correctly spaced and centred, these 

 doublets perform extremely well, and many here will doubtless 

 remember the exhibition l^y one of our members not long ago of 

 a fine Chevalier microscope furnished with a set of them. Triplet 

 combinations were also made, Holland's form (" Transactions of 

 Society of Arts," 1832) being, perhaps, the best known. It was 

 formed of three plano-convexes, the two lower in contact, with a 

 diaphragm between them and the upper and larger lens. By 

 this time, however, the introduction of flint and cix)wn combina- 

 tions, in which the aberrations were greatly reduced and the 

 aperture largely increased, gave a great imjDetus to the compound 

 microscope, and caused the simple to be comparatively neglected. 

 A notable improvement was later, however, made by Chevalier, 

 who superposed an achromatised concave over his doublet, which 

 gave a longer working distance, although it diminished the 

 size of the field. This convenient device appears to have been 



