148 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



was composed of two biconvex crown-glass lenses, and a biconcave 

 flint lens placed between them. 



C. Chevalier tells us 1 that between 1800 and 1810 M. Charles, of 

 the ' Institnt,' Paris, made small achromatic lenses; but they were 

 too imperfect to be of real service. In 1811 Fraimhofer made 

 achromatic doublets with no great success; and in 1823-4 an achro- 

 matic microscope was made by the Messrs. Chevalier, with four 

 doublet lenses arranged according to a plan devised by Selligue. 

 Their * Microscope d'Euler ' followed, and in 1827 Amici constructed a 

 horizontal microscope on achromatic principles, which was spoken 



well of. But while 

 up to a very recent 

 date it was common 

 to assert that the first 

 to suggest the plan 

 of combining two, 

 three, or four plano- 

 convex achromatic 

 doublets of similar 

 foci, one above the 

 other, to increase the 

 power and aperture, 

 was Selligue in 1823, 

 it is DOW known that 

 this had been antici- 

 pated by Marzoli (ch. 

 v. 353). Selligue's 

 plan was carried into 

 execution by the 

 Messrs. Chevalier. 

 The instrument em- 

 bodying tins plan is 

 shown in fig. 115. 



In a report to the 

 Academic Royale des 

 Sciences, the well- 

 known mathema- 

 tician Fresnel says, 

 concerning this mi- 







FIG. 115. Selligue's achromatic microscope (1823-4). 



croscope, that in comparing the objectives w r ith those of one of Adams's 

 best non-achromatic instruments that up to a magnification of 

 two hundred times Selligue's was decidedly superior ; but beyond 

 that magnification there was no superiority in the achromatic form, 

 and he preferred Adams's form for prolonged observations because 

 it gave a larger field than Selligue's. 



The mechanism of this microscope was similar to the English 

 model of Jones, shown at fig. 112. The focussing was by rack and 

 pinion acting on the stage, the pinion travelling with the stage on 

 the rack. Two draw-tubes, A and B, were applied within the 

 body-tube, C, the upper one having a biconcave lens, S, at the 

 1 Des Microscopes, Paris, 1839, p. sc,. 



