Chap. 9.] Newton* t Dijcovsry by means of the Prifm. 263 



glarTes, employed their thoughts a long lime on vari- 

 ous modes to bring the rays of light more accurately 

 to a focus. The greateft of 'philofophers, Newton 

 himfelf, was endeavouring to make a reflector of a 

 parabolical form, which, if he had fucceeded in his 

 attempt, would manifeftly have obviated this incon- 

 venience, when his thoughts were turned into another 

 channel by a difcovery, which taught him that the 

 errors arifmg from the fpherical form of his glafTes 

 were trifling, compared with what muft arife from his 

 newly difcovered property of the rays of light. Each 

 ray, notwithftanding the- exceeding minutenefs of its 

 breadth, was now found to be compounded of feven 

 other rays, from whofe various combinations all the 

 beautiful colours in nature originate. 



The experiment on which this difcovery is founded 

 is thus defcribed by Newton himfelf. 



In a very dark chamber, at a round hole F (Plate 

 XXIII. Fig. 50.) about one-third of an inch broad, 

 made in the fhut of a window, I placed a glafs prifm 

 ABC, whereby the beam of the fun's light, S F, 

 which came in at that hole, might be refracted up- 

 wards, toward the oppofite wall of the chamber, and 

 there form a coloured image of the fun, reprefented 

 at P T. The axis of the prifm (that is, the line paf- 

 fing through the middle of the prifm, from one end of 

 it to the other end, parallel to the edge of the refract- 

 ing angle) was in this and the following experiments 

 perpendicular to the incident rays. About this axis 

 I turned the prifm flowly, and faw the refracted light 

 on the wall, or coloured image of the fun, firft to 

 defcend, and then to afcend. Between the defcent 

 and afcent, when the image feemed ftationary, I ftop~ 

 ped the prifm and fixed it in that pofture. 

 . * Then I let the refracted light fall perpendicularly 

 S 4 upon 



