Achromatic compound Microscope. 251 



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 aperture, but which I ought to no- 

 tice 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 combination, 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 compound 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 combination ; 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. Al- 

 ready, 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 impediment to 

 the carrying defining power much farther. 



These statements are intended only as a notice, for practi- 

 cal purposes and wholly detached from theory, of facts that 

 have been in part very recently ascertained. In investigating 

 them I have depended chiefly on magnified measurements and 

 diagrams, which, though not strictly correct, may perhaps be 

 as well adapted as other far more exact but dillicult methods, 

 for constructing the minute and complex object-glass of a mi- 

 croscope. It is to this that the observations made particular- 

 ly apply ; and should they bring more within reach than pre- 

 viously, tin- requisites which have been enumerated, I trust 

 they may nol be unacceptable \<> the enlightened optician ; es- 



