THEORY OF LIGHT AND COLOURS. 79 



canary in this indance to produce a fingle new experiment ; 

 for of experiments there is already an ample dore, which are 

 fo much the more unexceptionable, as they rouft have been 

 conducted without the lead partiality for the fydein by which 

 they will be explained ; yet fome fads, hitherto unobferved, 

 will be brought forwards, in order to fhew the perfect agree- 

 ment of that iyftem with the multifarious phenomena of nature. 



The optical obfervations of Newton are yet unrivalled ; Excellence of 

 and, excepting fome cafual inaccuracies, they only rife in our fe^ons o?" 

 edimation, as we compare them with later attempts to im- Newton, 

 prove on them. A further confi deration of the colours of 

 thin plates, as they are defcribed in the fecond book of New- 

 Ion's optics, has converted that prepofleffion which I before 

 entertained for the undulatory fyftem of light, into a very 

 drong conviction of its truth and fuffjciency; a conviction 

 which has been fince mod ftrikingly confirmed, by an analyfis 

 of the colours of ftriated fubdances. The phenomena of thin Phenomena of 

 plates are indeed Co fmgular, that their general complexion i s thin P lates ' 

 not without great difficulty rcconcileable to any theory, how- 

 ever complicated, that has hitherto been applied to them ; and 

 fome of the principal circumdances have never been explained 

 by the moll gratuitous affumptions ; but it will appear, that the 

 minuted particulars of thefe phenomena are not only perfectly 

 confident with the theory which will now be detailed, but 

 ,that they are all the neceflary confequeuces of that theory, 

 without any auxiliary fuppodtions ; and this by inferences fo 

 fimplc, that they become particular corollaries, which fcarcely 

 require a didinct enumeration. 



A more extend ve examination of Newton's various writ- Newton firft 

 insrs has fhown me, that he was in reality the firft that fugged- fjjggefted the 



i r i ' i t n .» • • i i • theory ot undur 



ed men a theory as 1 mail endeavour to maintain ; that his own lation. 



opinions varied lefs from this theory than is now almod uni- 

 verfally fuppofed j and that a variety of arguments have been 

 advanced, as if to confute him, which may be found nearly in 

 a dmilar form in his own works ; and this by no lefs a mathe- 

 matician than Leonard Euler, whofe fyd^m of light, as far asEuler. 

 it is worthy of notice, either was, or mig(it have been, wholly 

 borrowed from Newton, Hooke, Huygeiis, and Malebranche. 



Thofe who are attached, as they may be. with the greated Reference to the 

 judice, to every doarine which is damped with the Newto- works of Ncw ~ 

 cian approbation, will probably be difpofed to bedow on thefe 



confi deration 



