and again in the same order ; so that he 
sometimes counted nine or ten of these cir- 
cles, the red immediately next to the pur- 
ple ; and the last colour that appeared be- 
fore the white was blue ; so that it began 
with red, and ended with purple. These 
rings, he says, would change their places, by 
changing the position of the eye, so that the 
glasses remaining the same, that part which 
was red in one position of the eye, was 
blue in a second, green in the third, &c. 
Sir Isaac Newton, having demonstrated 
that every dHferent colour consists of rays 
which have a different and specific degree of 
refrangibility, and that natural bodies ap- 
pear of this or that colour, according to 
their disposition to reflect this or that spe- 
cies of rays, pursued the hint suggested by 
tlie experiments of Dr. Hook, with regard 
to thin transparent substances. Upon com- 
pressing two prisms hard together, in order 
to make their sides touch one another, he 
observed, that in the place of contact they 
were perfectly transparent, which appeared 
like a dark spot, and when it was looked 
through, it seemed like a hole in that air, 
which was formed into a thin plate, by 
being impressed between the glasses. When 
this plate of air, by turning the prisms about 
their common axis, became so little inclined 
to the incident rays, tliat some of them be- 
gan to be transmitted, there arose in it 
many slender arcs of colours, which in- 
creased, as the motion of the prisms was 
continued, and bended more and more 
about the transparent spot, till they were 
completed into circles, or rings, surround- 
ing it; and afterwards they became continu- 
ally more and more contracted. He then 
took two object-glasses of a telescope, the 
one plano-convex, the other a little convex 
on both sides, he placed one of the faces of 
this upon the plane face of the former, and 
pressed the two glasses at first gently, and 
tlien, by degrees, more closely against one 
another. The effect of this gradual pressure 
was an appearance in the plate of air be- 
tween the glasses of different coloured cir- 
cles, which had the point of contact for the 
common centre, and which increased in 
number according to the greater degree of 
pressure, in such a manner that the circle 
which appeared last always surrounded the 
point of contact, and on a still further 
pressure extended its circumference while 
it contracted itself breadthwise, to form a 
kind of ring round a new circle that arose 
near its middle. The pressure having 
been carried to a certain terra, Newton 
stopped, and observed as follows. At the 
point of contact was a black spot that was 
encompassed by several series of colours. 
The order of the colours from the centre to 
the borders of the two glasses was this : in 
the first series, blue, white, yellow, and 
red ; in the second, violet, blue, green, 
yellow, and red ; in the third, purple, blue, 
green, yellow, and red ; in the fourth, green 
and red ; in the fifth, greenish blue and 
red ; in the sixth, greenish, blue, and pale 
red ; in the seventh, greenish blue, and 
reddish white. Beyond this number, the 
tints of which were regularly paler, the co- 
lour became white. Newton measured the 
diameters of the annular bands, formed of 
these different colours, by taking the points 
where they had most lustre ; and he found 
that the squares of those diameters were to 
one another as the terms of the ascending 
progression, 1, 3, 5, 7, 9, 11, &c. ; from 
which it results, that the intervals between 
the two glasses, relatively to the correspond* 
ing points, followed the same progression. 
From these proportions, it was merely ne- 
cessary to ascertain the absolute length of 
a single diameter, to know the lengths of all 
the others, as well as the different thickness 
of the plates of air at the points where the 
different colours were seen. He drew up 
a table of these degrees of thickness, by 
which it appears, that the most intense blue, 
for example, that of the first series, is ex- 
pressed by a thickness of 0.000024 of an 
inch, supposing the- visual ray to be nearly 
perpendicular to the two glasses. Sir Isaac 
Newton having measured also the diameters 
of the rings at the intermediate places where 
the colours were obscure, found that their 
squares were to one another as the even 
numbers 2, 4, 6, 8, 10, 1 2, &c. ; and hence 
tlie intervals bet#een the glasses, at the 
corresponding points, observed a similar 
progression. The diameters of the rings in- 
creased or diminished, as the visual ray was 
more or less inclined to the surface of the 
two glasses, so that the greatest contrac- 
tion took place when the eye was situated 
perpendicularly above the glasses. The 
diametets also retained the same propor- 
tions to one another. 
From other curious observations on these 
rings, made by different kinds of light 
thrown upon them, he inferred, that the 
thicknesses of the air between the glasses, 
where the rings are successively made, by^ 
the limits of the seven colours, red, orange, 
