q6o Mr. Brougham's Experiments and Observations on 
8. The last of these phenomena, which I shall mention, is 
the celebrated one observed by Sir Isaac Newton, namely, 
the rings of colours with which the focus of a concave glass 
mirror is surrounded. Sir Isaac made several most ingenious 
and accurate experiments to investigate their nature;* and 
finding their breadth to be in the inverse subdiydicate ratio of 
the mirror's thickness, he concluded that they were of the 
same nature and original with those of thin plates, described 
by him. ^ The Due de Chaulnes pursued these experiments 
with considerable success ; he found that the rings were 
brighter the nearer to the perpendicular the rays were inci- 
dent ; and that if, instead of a concave glass mirror, a metal 
one was used, with a small piece of fine cambric, or reticu- 
lated silver wire stretched before it, the colours were no longer 
disposed in rings, but in streaks, of the same shape with the 
intervals between the threads ; hence he concludes that they 
are owing to inflection ; that in passing through the first sur- 
face, they are inflected and condensed by the second. £ I am 
not, I own, quite satisfied with this account of the matter : 
that they are produced by inflection, the Duke's experiments 
put beyond doubt ; but that they should be formed in passing 
through the first surface, and reflected by the second, is quite 
inconsistent with the ratio observed by their breadth, this be- 
ing greater in the thinnest glass, and also with the order of 
the colours. Besides, all the coloured images which fall on 
the backside of the mirror, will be (by what we before found 
when speaking of flexibility §) reflected into a white focus ; so 
* Optics, Book II. Part IV. f Book II. Parts I. and II, 
I Mem. de VAcademie, pour Vannee 1755. 
§ Part II. Obs. 6 of this paper. 
