Dr. Young’s Lecture, &c. 
1 S 
The optical observations of Newton are yet unrivalled ; and, 
excepting some casual inaccuracies, they only rise in our esti- 
mation, as we compare them with later attempts to improve 
on them. A further consideration of the colours of thin plates, 
as they are described in the second book of Newton’s optics, 
has converted that prepossession which I before entertained for 
the undulatory system of light, into a very strong conviction of 
its truth and sufficiency; a conviction which has been since most 
strikingly confirmed, by an analysis of the colours of striated 
substances. The phenomena of thin plates are indeed so sin- 
gular, that their general complexion is not without great diffi- 
culty reconcileable 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 minutest particulars of 
these phenomena, are not only perfectly consistent with the 
theory which will now be detailed, but that they are all the 
necessary consequences of that theory, without any auxiliary 
suppositions ; and this by inferences so simple, that they be- 
come particular corollaries, which scarcely require a distinct 
enumeration. 
A more extensive examination of Newton’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 uni- 
versally supposed ; and that a variety of arguments have been 
advanced, as if to confute him, which may be found nearly in 
a similar form in his own works ; and this by no less a mathe- 
matician than Leonard Euler, whose system of light, as far 
as it is worthy of notice, either was, or might have been, 
