Life of the Duke de Chaulnes. 2G3 



dark room, if we receive a ray of light from the sun in the 

 axis of a olass, concave on one side, convex on the other, 

 and silvered on the convex side, this ray would necessardy 

 be reflected on itself ; but that if we opposed to the reflected 

 ray a white paper or pasteboard pierced in the middle to let 

 the direct ray pass through, the opening in the paper would 

 be circumscribed by four or five coloured rings. The duke 

 de Chaulnes, in repeating this experiment, found, by a most 

 fortunate chance, that when we dull the fore surface of the 

 glass by breathing on it from above, the rings, so far from 

 losing their distinctness, become more brilliant than before. 

 Nothing further was necessary to raise his curiosity. He 

 first thought to render this eflect permanent, by substituting 

 for the breath some water mixed with a little milk to dull 

 the olass; and he varied the experiment in so many difierent 

 ways, that at last he discovered the cause of this singular 

 phcenomenon to proceed from inflection ; that is, from the 

 property which rays of light have of bending at the ap- 

 proach of solid bodies. He found that the breath and the 

 water mixed with milk formed a kind of round net work, 

 which produced the appearance of coloured rings ; that, by 

 substituting for this kind of covering a clear muslin, there 

 are obtained, instead of rings, coloured squares or chequers : 

 and that parallel threads give bands or belts. In short, he 

 made such good use of this happy chance, that Newton's 

 experiment became in his hands an object altogether new, 

 and far more interesting than it had hitherto been. 



Whilst the duke de Chaulnes was amusing himself with 

 his dioptric experiments and improvements, he applied 

 himself to another object equally important : this was the 

 perfecting of astronomical instruments, or, to propose the 

 problem in its fullest extent, the art of producing from in- 

 struments of a very small radius, an equal degree of accu- 

 racy at least with that obtained from those of a considerable 

 radius, such as were then commonly used. This problem, 

 on account of its dilliculty, might till then have been ranked 

 with those of the trisection of an angle, and the quadrature 

 of the circle-, but experience has proved to us that he was 

 fully competent to resolve it. 



R 4 The 



