CHROMATICS. 
thicknefs increased in a certain ratio, according to the dis¬ 
tance from the point of contaCl. 
He preifed thefe glaffes (lowly together, by which means 
the colours very loon emerged, and appeared diftmCt to a 
conliderable diilance; next to the pellucid central fpot 
made by the contact of the glades, Succeeded blue, yel¬ 
low, white, yellow, and red. The blue was very little in 
quantity, nor could he difcern any violet in it; but the 
yellow and red were very copious, extending about as 
far as the white, and four or live times as far as the blue. 
The next circuit immediately lurrounding thefe confided 
of violet, blue, greerj, yellow, and-red; all thefe were 
very copious, except the green, which was very little in 
quantity, and feemed more faint and dilute than the other 
Colours. The third circle of colours was purple, blue, 
gieen, yellow, and red; in this the purple was more red- 
dilh than the violet in the former circuit, and the green 
was more confpicuous, being as bright and copious as any 
of the other colours, except the yellow; the red was alio 
fomewhat faded. The fourth circle confided of green and 
red; the green was copious and lively, inclining on one 
fide to blue, on the other to yellow, but there was neither 
violet, blue, nor yellow ; and the red was very imperfeCf. 
Rach outer circuit, or ring, was more obfcure than thole 
within, like the circular waves upon a didurbed (heet of 
water, till they at lad ended in perfeft whitenefs. 
As the colours were thus found to vary according to the 
diderent diitances of the glafs-plates from each other, fir 
Il'aac judged that they proceeded from the different thick¬ 
nefs of the plate of air, intercepted between the glades; 
and that this plate was, by the mere circumdance of thin- 
nefs or thicknefs, difpofed to refleCl or tranfmit this or that 
particular colour 5 from whence he concluded, as before 
obferved, that the colours of all natural bodies depended 
on their component particles. He alfo conliruCled a ta¬ 
ble, wherein the thicknefs of a plate, neceffary to refieft 
any particular colour, was expreffed in parts of an inch, 
divided into i,oeo,ooo parts. 
It has been already obferved, that the thin plates, made 
ufe of in the different experiments, reflected fome kinds 
of rays in particular parts, and tranlmitted others in the 
fame parts. Hence the coloured rings appeared varioufly 
difpofed, according as they were viewed by reflected or 
traufmitted light; that is, according as the plates were 
or were not held up between the eye and the window. 
That we may underltand this better, the following table 
has been formed. On one fide are mentioned the colours 
appearing on the plates by reflected light, and on the 
other thofe which are perceptible when the glades are 
held betweeen the eye and the window. The centre, 
when the -glaffes are in full contaCt, is perfectly tranlpa- 
rent; this ipot, therefore, when viewed by reflected light, 
appears black, becaufe it tranfmits all the rays; and for 
the lame reai'on it appears white, when viewed by tranl¬ 
mitted light. 
Colours by Reflect’d Light* 
Colours by Tranfmitted Light 
Black 
White . 
Blue 
Yellowiflr-red 
White 
Black 
Yellow 
Violet 
RVd 
Blue 
Violet 
White 
Blue 
Yellow 
Green 
Red 
Yellow 
Violet 
Red 
Blue 
Purple 
Green 
Blue 
Yellow 
Green ~i 
Yellow > 
Red 
Red 3 
Blueilh-gre^i 
Green 
Red 
Red 
Blueilh-green 
Greenifli-blue 
Red 
VOL. IV. No, 216. 
Red 
525 
In comparing the rings produced by tranlmitted with 
thofe produced by reflected light, the white is found op- 
pofed to the black, the red to the blue, the yellow to the 
violet, and the green to a colour compofed of red and 
violet; in other words, the parts of the glal's, which when 
looked at are white, appear black on looking through the 
glafs; and, on the contrary, thofe which appear black in 
the firlt inftance, appear white in the lecond ; and fo of 
the other colours. Newton has (hewn, that the rays of any 
particular colour are difpofed to be refle&ed, when the 
thicknefies of the plate of air are as the numbers 1,3, 5, 
7, 9, 11, &c. and that the fame rays are difpofed to be 
tranlmitted at the intermediate thicknefies, which are as 
the numbers o, 2, 4, 6, 8, 10, &c. 
The places of reflection or tranfmiflion of the feveral 
colours in a l'eries, are fo near each other, that the colours 
dilute each other by mixture; whence the number of ie« 
ries, in the open day-light, feldom exceeds feven or eight. 
But if the fyflem be viewed through a prifm, by which 
means the rings of various colours are i'eparated, accord¬ 
ing to their refrangibility, they may be feen on that fide 
towards which the refraftion is made, fo numerous that 
it is impoflible to count them. Or, if in a dark chamber 
the fun’s light be feparated into its original rays, by 3 
prifm, and a ray of one uncompounded colour be received 
upon the two glaffes, the number of circles will become 
very numerous, and both the refleCled and tranfmit ted 
light will remain of the fame colour as the original inci¬ 
dent ray. This experiment fifiews, that in any feri.es, the 
circles formed by the lefs refrangible rays exceed, in mag¬ 
nitude, thofe which are formed by the more refrangible j 
and, confequently, that, in any feries, the more refran¬ 
gible rays are reflected at lefs thicknefies than thofe which 
are lefs refrangible. 
If we apply water to the edges of the glafs, it will be 
attracted between them; and, filling all the intercedent 
lpace, it will become a thin plate of the fame dnnenfions 
as that which before was conllituted of air ; in this cafe, 
the circular rings grow lefs, and the colours fainter, but 
not varied in lpecies. They become contracted in dia¬ 
meter, nearly in proportion of 7 to 8, and confequently, 
the intervals of the glaffes, at fimilar circles, as caufed by 
thefe two mediums, are as about 3 to 4; that is, as the 
iines of refraction out of water into air. 
We have already fpoken of the variety of colours pro¬ 
duced by bubbles blown in foap-water; but, as thefe co¬ 
lours are commonly too much agitated by the external 
air to admit of any certain obfervation, it is neceflary to 
cover the bubble with a clear glafs, in which fituation 
the following appearances take place: the colours emerge 
from the top of the bubble, and as it grows thinner, by 
the fubfidence of the water, they dilate into rings parallel 
to the horizon, which defcend (lowly, and vanilh luccel- 
fively, at the bottom. This emergence continues till the 
water at the upper end of the bubble becomes too thin 
to refleCt the light, at which time a circle of an intenfe 
biacknefs appears at the top, which (lowly dilates, fome- 
times to three quarters of an inch in breadth, before the 
bubble breaks. Reckoning front the black central (pot, 
the reflected colours are the fame, in fucceflion and qua¬ 
lity, as thofe produced by the afore-mentioned plate of 
air3 and the appearance of the bubble, it viewed by.trans¬ 
mitted light, is fimilar to that of the plate of air, in like 
circumftances. 
If we take very thin plates of talc, or Mufcovy glafs, 
that exhibit tlieie colours ; then, by wetting the.plates* 
the colours remain as before, bur become more faint and 
languid, efpecially when wetted on the under fide. So 
that the thicknefs of any plate, requilite to produce any 
colour, feems to depend only on the denfity of the plate, 
and not on the denfity of the inclofing medium. But ths 
colours are more vivid, as their denfities are different. 
If two pieces of plate-glafs, or even common glafs, be 
previoully wiped, and then rubbed together, they will 
loon adhere, with a confiderable degree of force, and ex¬ 
hibit various ranges of colours, much broader than thole 
6 § obtained 
