On the Liflex'ion, Reflexion, and Colours ef Light. j;3 



prifm, T placed a black unpolifhed pin (whofe diameter was everywhere one tenth of an 

 inch) parallel to the chart and in a vertical pofition. Its fliadowwas formed in the fpec- 

 trum on the chart, and had a confiderable penumbra, efpecially in the brighteft red, for it 

 was by no means of the fame thicknefs in all its parts ; that in violet was broadeft and moft 

 diftinft ; that in the red narrowed and moft confufed ; and that in the intermediate co- 

 lours was of an intermediate thicknefs and degree of diflin£\nefs. It was not bounded by 

 ftraight but by curvilinear fides, convex towards the axis, to which they approached as to an 

 afymptote, and that neareft in the lead refrangible rays, as is reprefented in Fig. 2, where 

 A B is the axis, I K L M N A, and H G F E D A the two outlines. Nor could this 

 be owing to any irregularity in the pin, for the fame thing happened in all forts of 

 bodies that were ufed ; and alfo if the prifin was moved on its axis, fo that the colours 

 might afcend and defcend on thefe bodies ; dill, wherever the red fell, it made the leaft, 

 and the violet the greateft fhadow. 



Ohf. II. In the place of the pin I fixed a fcreen, having in it a large hole, on which was 

 a brafs plate pierced with a fmall hole -^d of an mch in diameter. Then caufing an afllftant 

 to move the prifm flowly on its axis, I obferved the round image made by the different 

 rays pafTmg through the hole to the chart ; that made by the red was greateft, by the violet 

 leaft, and by the intermediate rays, of an intermediate fize. Alfo, when at the back of the 

 hole, I held a fharp blade of a knife, fo as to produce the fringes mentioned by Grimaldo 

 and Newton ; thofe fringes in the red were broadeft and moil moved inwards towards 

 the fliadow, and moft dilated when the knife was moved over the hole, and the hole 

 itfelf on the chart was more dilated during the motion when illuminated by the red, than 

 when illuminated by any other of the rays, and leaft of all when illuminated by the violet. 

 Now in Obfervation I. the angle of incidence of the red rays was equal to that of the violet 

 and all the reft, and yet the angle of inflexion was greateft and leaft in the violet ; and in- 

 deed the dlflerence between the two was greater than appears at firft from the experiment ; 

 for that part of the fhadow which was formed by the violet, fell at a gre ter diftance from 

 the point of incidence than did that part which was formed by the red, from the diver- 

 gency of the di/Terent rays upwards by the refraflion, as appears in Fig. 3, where D E is 

 the window, F G the beam propagated through the hole F, refradled by the prifm 

 K I II and pointing on the chart O P q s ; the fpedlrum v r, being feparated in L r, the 

 red rays incident on the pin C D at C ; and M v, the violet incident at D ; the rtiadow of 

 D C being formed in v r, fo that v being farther from D, than r is from C, therefore (by 

 the propofitions formerly laid down) the fliadow in v fliould be confiderably lefs than in r, 

 if the rays were equally inflefled. Laftly, in Obf. II. the angle of the red's incidence was 

 nearly equal to that of the violet's, by the motion of the prifm, and the confequent motion 

 of the colours ; only that if there was any difference it was on the fide of the violet : and 

 yet the violet was leaft infleiSled, and the red moft inflc£led ; and fo of the fecond inflexion 

 by the knife-blade : wherefore I conclude that the rays of the fun's light diflcr in degree of 

 inflexibility, and that ihofe which are leaft refrangible are moft inflexible. 



f)bf. HI. My room being darkened as before, and a conical beam propagated througii 



the fmall hole in the window fliut ; at this hole I placed a hollow prifm made of broken 



plates of mirror, and of fuch an angle that, when filled with diftilled water, it caft a fpcc- 



VoA. I. — March 1798. 4B trum. 



