ACHROMATIC COMPOUND MICROSCOPES. 
199 
under-corrected pencil from the front glass, and to counteract its error by 
over-correction in the middle one. 
Slight errors in colour may in the same manner be destroyed by opposite 
ones ; and on the principles described, we not only acquire fine correction for 
the central ray, but by the opposite effects at the two foci on the transverse 
pencil, all coma can be destroyed, and the whole field rendered beautifully flat 
and distinct. 
The occurrence of two good combinations among the numerous ones of 
Utzschneider’s glasses will now appear only what might be expected ; and 
more are to be obtained from them by varying the distance of the glasses from 
each other. 
I have very lately seen one of Chevalier’s combinations of three glasses, 
each about 0.4 inch in focus, inferior indeed to these, and of contracted aper- 
ture, but which I ought to notice because its image is pretty well corrected 
with the glasses in the order given to them by the maker. At the same time, 
I do not suppose him to be acquainted with the principle which constitutes the 
key to the effect ; otherwise he would hardly have failed to apply it more 
effectually. 
The achromatic meniscus that has been described enters well into combina- 
tion, and may perhaps be useful as the front glass of three, but its power is 
small in proportion to its curves. It admits too of being cemented at the back 
of an achromatic plano-convex ; and they may make together a powerful com- 
pound glass to go before another. 
Though the plano-concave form has been proposed for the flint lens, in the 
larger glasses it might perhaps occasionally be relinquished with benefit. 
Some attention has yet to be given to obtain the best effect from combina- 
tion ; so that with the largest pencil that may be found desirable, and sufficient 
clear space before the front of the glass, the field may be aplanatic, and the 
focus short. Already, however, the three first plano-convex glasses that have 
been made for me by Tulley, only for preliminary experiments, the shortest 
of them of 0.7 inch focus, have produced at an aperture of 50° the most distinct 
microscopic vision that I have yet met with ; and I anticipate no serious im- 
pediment to the carrying defining power much further. 
These statements are intended only as a notice, for practical purposes and 
