in the Seventeenth Century. \ 5 



" Colour/' he says in another place,* " is nothing but the 

 disturbance of light by the communication of the pulse to other 

 transparent mediums, that is by the refraction thereof." His 

 precise hypothesis regarding the different colours wasf "that 

 Blue is an impression on the Retina of an oblique and confus'd 

 pulse of light, whose weakest part precedes, and whose 

 strongest follows. And, that red is an impression on the Retina 

 of an oblique and confus'd pulse of light, whose strongest part 

 precedes, and whose weakest follows." 



Hooke's theory of colour was completely overthrown, within 

 a few years of its publication, by one of the earliest discoveries 

 of Isaac Xewton (b. 1642, d. 1727). Newton, who was elected 

 a Fellow of Trinity College, Cambridge, in 1667, had in the 

 beginning of 1666 obtained a triangular prism, " to try- 

 therewith the celebrated Phaenomena of Colours." For this 

 purpose, " having darkened my chamber, and made a small hole 

 in my window-shuts, to let in a convenient quantity of the 

 Sun's light, I placed my Prisme at his entrance, that it might 

 be thereby refracted to the opposite wall. It was at first a 

 very pleasing divertisement, to view the vivid and intense 

 colours produced thereby ; but after a while applying myself to 

 consider them more circumspectly, I became surprised to see 

 them in an oblong form, which, according to the received laws 

 of Refraction, I expected should have been circular" The 

 length of the coloured spectrum was in fact about five times as 

 great as its breadth. 



This puzzling fact he set himself to study ; and after more 

 experiments the true explanation was discovered namely, 

 that ordinary white light is really a mixture of rays of every 

 variety of colour, and that the elongation of the spectrum is 

 due to the differences in the refractive power of the glass for 

 these different rays. 



" Amidst these thoughts," he tells us,+ " I was forced from 



*To the Royal Society, February 15, 1671-2. 



t Micrographia, p. 64. 



J Phil. Trans., Xo. 80, February 19, 1671-2. 



