Discovery of Ids Theory of Colours. 115 



various writings has shown me that he was in reality the first 

 who suggested such a theory [the undulatory theory of light], 

 as I shall endeavour to maintain ; that his own opinions varied 

 less from this theory than is now almost universally supposed ; 

 and that a variety of arguments have heen advanced, as if to 

 confute him, which may be found in a similar form in his own 

 works ; and this by no less a mathematician than Leonard 

 Euler, whose system of light, as far as it is worthy of notice, 

 either was, or might have been, wholly borrowed from Newton, 

 Hooke, Huyghens, and Malebranche. 



" Those who are attached, as they may be with the greatest 

 justice, to every doctrine which is stamped with the Newtonian 

 approbation, will probably be disposed to bestow on these con- 

 siderations so much the more of their attention as they shall 

 appear to coincide more nearly with Newton's opinion. For 

 this reason, after having briefly stated each particular position 

 of my theory, I shall collect from Newton's various writings 

 such passages as seem to be the most favourable to its admis- 

 sion ; and although I shall quote some papers which may be 

 thought to have been partly retracted at the publication of the 

 Optics, yet I shall borrow nothing from them that can be sup- 

 posed to militate against his maturer judgment." 



The fact that Young, the founder of the undulatory theory 

 of light, in this Bakerian Lecture, in which it has been said 

 that he laid the foundations of that doctrine, should set forth 

 his views in a series of postulates followed by citations from 

 the writings of Newton to give them weight and proof, may 

 justly surprise those who have trusted to the second-hand in- 

 formation derived from carelessly compiled text-books and 

 from hastily prepared popular lectures. But then where 

 would be the pugilistic charm of the popular lecturer on the 

 undulatory theory of light if Newton, his champion, the vio- 

 lent defender of the emanation cause, should decline to enter 

 as a contestant ? 



Under the heading of Hypothesis III. of this paper we first 

 meet Young's theory of colour-sensation. 



" Hypothesis III. The Sensation of different Colours depends 

 on the different frequency of Vibrations, excited by Light in the 

 Retina, 



Passages from Newton. 



" The objector's hypothesis, as to the fundamental part of 

 it, is not against me. That fundamental supposition is, that 

 the parts of bodies, when briskly agitated, do excite vibrations 

 in the ether, which are propagated every way from those 

 bodies in straight lines, and cause a sensation of light, by beat- 

 ing and dashing against the bottom of the eye, something after 



