728 PHILOSOPHICAL TRANSACTIONS. [anno 17 q6. 



used ; and also if the prism was moved on 'its axis, so that the colours might 

 ascend and descend on these bodies, still wherever the red fell it made the least, 

 and the violet the greatest shadow. 



Oh. 1. In the place of the pin, I fixed a screen, having in it a large hole on 

 which was a brass plate, pierced with a small hole Vt of an inch in diameter; 

 then causing an assistant to move the prism slowly on its axis, I observed the 

 round image made by the different rays passing through the hole to the chart ; 

 that made by the red was greatest, by the violet, least, and by the intermediate 

 rays of an intermediate size. Also when at the back of the hole I held a sharp 

 blade of a knife, so as to produce the fringes mentioned by Grimaldo and 

 Newton; those fringes in the red were broadest, and most moved inwards to 

 the shadow, and most dilated when the knife was moved over the hole ; and the 

 hole itself on the chart was more dilated during the motion when illu- 

 minated by the red than when illuminated by any other of the rays, 

 and least of all when illuminated by the violet. Now, in Obs. 1, the angle 

 of incidence of the red rays was equal to that of the violet, and all the 

 rest, and yet the angle of inflection was greatest, and least in the violet ; 

 and indeed the difference between the two was greater than appears at 

 first from the experiment ; for that part of the shadow which was formed by the 

 violet fell at a greater distance from the point of incidence, than did that part 

 which was formed by the red, from the divergency of the different rays upwards 

 by the refraction, as appears in fig. 3 ; where de is the window, fg the beam 

 propagated through the hole f, refracted by the prism kih, and painting on the 

 chart ovqs ; the spectrum vr being separated into Lr the red rays incident on the 

 pin CD «t c, and mv the violet incident at d ; the shadow of dc being formed in 

 vr, so that v being farther from D than r is from c, therefore, by the proposi- 

 tions before laid down, the shadow in v should be considerably less than that in 

 r, if the rays were equally inflected. Lastly, in Obs. 2, the angle of the red's 

 incidence was nearly equal to that of the violet's, by the motion of the prism, 

 and the consecjuent motion of the colours; only that, if there was any differ- 

 ence, it was on the side of the violet ; and yet the violet was least inflected, and 

 the red most inflected ; and so of the 2d inflection by the knife blade : I there- 

 fore conclude that the rays of the sun's light difler in degree of inflexibility, and 

 that those which are least refrangible are most inflexible. 



Oks. 3. My room being darkened as before, and a conical beam propagated 

 through the small hole in the window-shut ; at this hole I placed a hollow prism, 

 made of broken plates of mirror, and of such an angle, that when filled with 

 distilled water, it cast a spectrum on an horizontal table, and was there received 

 on a chart 7 feet from tiie window. I then placed on the same table, and in 

 the rays between the chart and the prism, at 3 inches from the chart, two sharp 

 knifc-bhides with even edges, and fixed to a board vv\th wax, so as to make an 



