172 Transactions of the Society. 



correction of the ocular compensating for want of correction of the 

 objective. His plan was not, however, successful, and the first 

 practical achromatic Microscope was not produced until lifty years 

 after his time. 



Described and figured in Petri's " Das Mikroskop," p. 1 62. 

 (See Journ. E.M.S., 1909, p. 658.) 



The instrument is not in working order, the lens in the eye- 

 piece being absent. Doubtfully achromatic. 



20. Charles Chevaliers "Microscope Achromatiqiie " 



(c. 1834). 



The brothers Chevalier, of Paris, were the first opticians to 

 produce, about 1823, practically useful achromatic object-glasses 

 for the Microscope. The present instrument is an early specimen 

 made by Charles Chevalier. Tlie mechanical model Ibllowed is 

 still that of -Jones's " most improved," with various modifications. 

 The arm carrying both the body and the limb is fixed by a 

 compass-joint to tlie top of the stem, which itself is supported on 

 a flat solid tripod. Focusing is effected by rack-and-pinion to 

 the stage, which itself is mechanical in one direction only. 



Described and figured in Chevalier's " Des Microscopes et de 

 leur Usage," 1839, pp. 98-100, pi. 3. (See Journ. E.M.S., 1909, 

 p. 658.) * ■ 



The advance in definition is very marked, and when a balsam- 

 mounted specimen is used the improvement is still more marked. 

 Balsam mounts were, to all intents and purposes, unknown at this 

 period, though Swammerdam (1637-1680) used balsam to render 

 his anatomical preparations transparent. An account of his 

 method is to be found at the end of Schrader's " Observationes et 

 Historic " (Amsterdam, 1674). See also L. C. Miall, op. cit. 

 p. 178. 



21. Charles Chevalier (c. 1840). 



This exceedingly well-made instrument is an enlarged and 

 improved Microscope upon the model of the preceding, embodying 

 a number of devices for use in an erect or in a horizontal position, 

 and for the observation of chemical reactions. It is described by 

 its maker as his " Microscope Achromatique Universel." The 

 focusing arrangements, both coarse and fine, are still attached to 

 the stage, whilst the body remains fixed. In the horizontal 

 position a right-angled prism is used for deflecting the rays into 

 the tube. The mirror is plane and concave, and is movable by 

 rack-and-pinion. The whole Microscope is exceedingly steady, 

 and all the motions very smooth. 



