614 



ON THE THEORY OF LIGHT AND COLOURS. 



I before entertained for the undulatory sys- 

 tem of iightj into a very strong conviction of 

 its truth and sufficiency ; a conviction which 

 has been since mosi striiiingly contirmed, by 

 an analysis of the colours of striated sub- 

 stances. The phenomena of thin plates are 

 hideed so singular, that their general com- 

 plexion is not without great difficulty recon- 

 cileable to any theory, however complicated, 

 that has hitherto been applied to them ; and 

 some of the principal circumstances have 

 never been explained by the most gratuitous 

 assumptions ; but it will appear, that the mi- 

 nutest particulars of these phenomena are not 

 only perfectly consistent with the theory, 

 which will now be detailed, but that they are 

 all necessary consequences of that theory, 

 without any auxiliary suppositions: and this 

 by inferences so simple, that they become 

 particular corollaries, which sciucely requue 

 a distinct enumeration. 



A more extensive examination of New- 

 ton's various writings has shown me, that he 

 was, in reality, the first that suggested such a 

 theory as I shall endeavour to maintain ; 

 that his own opinions varied less from this 

 theory, than is now almost universally sup- 

 posed ; and that a variety of arguments have 

 been advanced, as if to confute hmi, which 

 may be found nearly in a similar form in his 

 own works ; and this, by no less a mathema- 

 tician 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, Huygens, and Male- 

 branche. 



Those who are attached, as they may be 

 with the greatest justice, to every doctrine 

 which is stamped with the Newtonian ap- 

 probation, will probably be disposed to be- 



stow on these considerations 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, 1 

 shall collect, from Newton's various writings, 

 such passages as seem to be the most favour- 

 able to its admission ; 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 supposed to militate 

 against his maturer judgment. 



Hypothesis i. A luminiferous Ether per- 

 tades the Universe, rare and elastic in a hiTh 

 degree. 



PASSAGES FKOM NEWTON. 



'i The hypothesis certainly has a much 

 greater affinity with his o«n," that is. Dr. 

 Hooke's, " hypothesis, than he seems to be 

 aware of; the vibrations of the ether being 

 as useful and necessary in this, as in liis." 

 (Phil. Trans. VII. 5087. Abr. I. 145. Nov. 

 1672.) 



"To proceed to the hypothesis: first, it 

 is to be supposed therein, that there is an 

 ethereal medium, much of the same consti- 

 tution with air, but far rarer, subtler, and 

 more strongly elastic— It is not to be sup- 

 posed, that this medium is one uniform mat- 

 ter, but compounded, partly of the main 

 phlegmatic body of ether, partly of other 

 various ethereal spirits, much after the man- 

 ner that ah- is compounded of the phlegma- 

 tic body of air, intermixed with various va- 

 pours and exhalations: for the electric and 

 magnetic effluvia, and gravitating principle, 

 seem to argue such variety." (Birch Hist. 

 R. S. HI. 249. Dec. 1675.) 

 " Is not the heat (of the warm room) con- 



