148 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



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

 Hint lens placed between them. 



( '. ( Ihevalier tells us ' that between 1800 and 1810 M. Charles, of 

 the ' Institiit." Paris, made small achromatic lenses; but they were 

 too imperfect in lie of real service. In 1811 Fraunhofer 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 Amid constructed a 

 hori/.ontal micro-cope 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, 

 thrive, oi' four plano- 

 convex achromatic 

 doublets of similar 

 foci, one above the 

 other, to increase the 

 power and aperture, 

 was Selliguein l-j:!. 

 it is now known that 

 this had been antici- 

 pated b\ Mar/oli (ch. 

 v. 353). Selligue's 

 plan was carried into 

 execution 1 >y the 

 Messrs. Chevalier. 

 The instrument em 

 bodvinir this plan is 



115. 



In a report to the 

 Academic Royale des 

 Sciences, the well- 

 known mathema- 

 tician Fresnel says, 

 c incerniny this mi- 



shown in fig 



FIG. 115. Selligue's arhnmuitic microM-. .|M' i 



823 



V ''llV^AAAAAAi^ I 1 1 1 ,-> Jill" 



uscope. that in comparing the objectives with those of one of Adams's 

 """ achromatic instruments that un to a magnification of 



up 



"" hundred times Selligue's was decidedly 

 that 



superior ; but beyond 



magnification there was no superiority in the achromatic form. 

 '" preferred Adams's form for prolonged observations because 

 '< .-i larger lie],! i | l;m Selligue's. 



' chanism ,,(' t|,js mici-oscope was similar t.o the English 



shown ai tig. I \-2. The focussing was by rack and 



on the stage, the pinion travelling with the stage on 



1 draw tubes A and 15. were applied within the 



t1 "' upper one having a biconcave lens. S, at the 



pee, I'.-n-is, ls:;:i, ,,. 86. 



