*66 , Life of the Duke de Chaidnes. 



mounted on a kind of micrometers, which measure the mo- 

 tion of the instrument to the four hundredth part of a Hne. 

 It v,'as by the help of these microscopes and several other 

 ingenious instruments, that he measured the exact degree 

 of refrangibility of different kinds of glass ; the convex and 

 concave cui'valure of all the pieces of an object glass without 

 separating them from each other. This singular problem 

 nppears at first sight impossible to resolve, or to determine 

 even with any degree of precision when the curvature of the 

 glr.ss which is to correct the aberration of refrangibility, .and 

 tlestroy the colours, is such as it ought to be. 



These discoveries were followed by the invention of a new 

 parallactic instrument more firm and convenient than those 

 already in use, by several reflections on the manner of ap- 

 plying the n)icrometer to these telescopes, and to measure 

 accurately the value of the parts of this instrument. For 

 ttiis purpose he used a wall, which, that it might be seen 

 from a distance, had the singular property of presenting to 

 the observer thick strokes, by means of which he could 

 measure very small intervals. This Memoir shines through- 

 out with the genius of the inventor, and cannot be read 

 without admiring his contrivances to avoid difficulties, which 

 at the first glance appear insurmountable. Unfortunately 

 this was his last work, and it niakes us greatly regret that 

 others which he proposed to execute on this subject, were 

 prevented from being finished by his death. 



The duke de Chaulnes's disposition was of the most 

 arniable kind : the mildness from which it proceeded was 

 ornamented by the greatest politeness, and he could scarcely 

 reckon anvbut friends among all those with whom he lived. 

 The king himself was well acquainted with his zeal and his 

 luerit, and often showed him the slrongest marks of that 

 kindness which gives pleasure to a good subject and happi- 

 ness to a gentleman of the court. From his elevated situa- 

 tion one would suppose he was out of the reach of chagrin 

 ?,nd reverses, but iie experienced some of the most unplea- 

 sant ones for a considerable time. lie however opposed to 

 them the constancy prescribed by philosophy, and the pa- 

 tience 



