VOL. XXXII.] PHILOSOPHICAL TRANSACTIONS. ^ ()07 



On the (liferent Refrangibility of coloured Light. By the Rev. J. T. Desa- 

 guliers, LL. D. F. R. S. N° 374, p. 206. 



Sir Isaac Newton, in his Optics, B. 1, Prop. 1, Exp. 2, relates an experi- 

 ment made with a card, or paper, painted red on one half and blue on the 

 other ; which being enlightened by a candle, the image, by the interposition 

 of a lens, is so projected on a white paper, held on the other side of the lens, 

 that the place where the blue half appears distinct, or as the opticians term 

 it, the distinct base of the image of the blue half, is much nearer to the lens 

 than the place of the image of the red half. And this is made apparent, by 

 seeing on one of these images the representation of the black threads, wrapped 

 round the card, while they are not visible on the other. This is fully described 

 in the place abovementioned ; but yet a gentleman* abroad has called the expe- 

 riment in question, and denied the matter of fact, saying, that he could not 

 make it succeed, but proposes an experiment of his own, to disprove the dif- 

 ferent refrangibility of the rays. 



On this I was desired to make the experiment over again, before the Royal 

 Society, which succeeded well. But because that care must be taken in making 

 it, I shall mention all the particulars observed in the performance; which, if 

 duly put in practice, will make the experiment always succeed. 



I painted one half of the card eb, fig. 17, pl. 14, as b, with ultramarine, 

 made deeper with a small mixture of indigo, and the other half r, I painted 

 over with cinnabar heightened with a little carmine; so that the line that 

 separated the red from the blue, was perpendicular to the long sides of the 

 cord. I then wrapped a black silk four times together, over the middle of 

 each painted part of the card, as in fig. 18. On a square trencher, fig. ig, 

 painted black, and suspended vertically against a wall, I fixed my coloured 

 card with a pin; and the room being made very dark, I enlightened the card 

 with a strong light thrown on it from a dark lantern, having two convex 

 glasses in it; then setting up the lens ll, represented by fig. 20, in such a 

 manner, that its axis passed perpendicularly through the image of the card, 

 and at the distance of Q feet from it, the image of the card being received on 

 a white paper, at the distance of 9 feet on the other side of the lens, at b, the 

 blue half appeared distinct, with the image of the black silk going vertically 

 along its plain, while no appearance of the black silk was perceivable on the 

 red half. Then removing the paper about 2 inches, to r, the red half of the 

 image had a black-line very plain upon it, while it was invisible on the blue 



* Act. Erudit. Ijps. Supplem. torn. 8, §. 3, p. 130, 131. 



