CHROMATICS. 
hot merely a superior but a divine charac- 
ter. Upon these several facts I ground my 
belief as a Christian. And, till the evi- 
dence on which they rest can be invali- 
dated by counter evidence, I must retain 
my principles and my profession.” 
CHROMATICS, is that part of optics 
which explains the several properties of the 
colonrs of light, and of natural bodies. 
Before the time of Sir Isaac Newton, the 
notions concerning colour were very vague. 
Res Cartes accounted colour a modifica- 
tion of light ; and he imagined that the dif- 
ference of coloiir proceeds from the preva- 
lence of the direct of rotatory motion of 
the particles of light. Grimaldi, Dechales, 
and many others, imagined that the ditfer- 
ences of colour depended upon the quick 
or slow vibrations of a certain elastic me- 
dium with which the universe is filled. Ro- 
liault conceived, that the different colours 
were made by tiie rays of light entering the 
eye at different angles vshfli respect to the 
optic axis. And Dr. Hooke imagined that 
colour is caused by the sensation of the ob- 
lique or uneven pulse of light ; which being 
capable of no more than tWo varieties, he 
conchrded there could be no more than two 
primary colours. 
Sir Isaac Newton, in the year 1666, be- 
gan to investigate tliis subject ; when finding 
that the coloured image of the sun, formed 
by a glass prism, was' of an oblong, and 
not of a circular form, as, according to tlie 
laws of equal refraction, it ought to be, he 
conjectured tliat liglit is not homogeneal ; 
but that it consists of rays of different co- 
lours, and endued with divers degrees of 
rtfrangibility. And, from a farther prose- 
cution of his experimenfs, he concluded 
that the different colours of bodies arise 
from their reflecting this or that kind of 
rays most copiously. This method of ac- 
counting for the different colours of bodies 
soon became generally adopted, and still con- 
tinues to be the most prevailing opinion. It is 
hence agreed that the liglit of the sun, which 
to us seems white and perfectly homogeneal, 
is composed of no fewer than seven differ- 
ent colours, i-iz. red, orange, yellow, green, 
bine, purple, and violet or indigo ; that a 
bndy wiiicli appears of a re'd colour has the 
jiropei'ty of reflecting the red rays more 
plentifully than the rest ; and so of the 
other colours, the orange, yellow, green, 
&c. : also that a body w hich appears black, 
instead of reflecting, absorbs all or the 
nrost part of the rays that tall upon it ; 
while, on the contrary, a body which ap- 
pears white, reflects the greater part of all 
the rays indiscriminately, without separating 
them one from another. 
The foundation of a rational theory of 
colours being thus laid, the next inquiry 
was, by what peculiar mechanism, in the 
structure of each particular body, it was fit- 
ted to reflect one kind of rays more thart 
another, and this is attributed by Sir I. New- 
ton, to the density of these bodies. Dr. 
Hooke had remarked, that thin transparent 
substances, particularly soap-\yafef blown 
into bubbles, exhibited various colours, ac- 
cording to their thinness j and yet, when 
they have a considerable degree of thick- 
ness, they appear colourless. And Sir 
Isaac himself had observed, that as he vvas 
Compressing two prisms hai'd together, in 
order to make their sides (which happefied 
to be a little convex) to touch one another, 
iu' the place of contact they were both per- 
fectly transparent, as if they had been but 
one continued piece Of glass : but round 
the point of contaict, where the glasses were 
a' little separated from each other, rings of 
different colours appeared. And when he 
afterwards, farther to elucidate this matter, 
employed two convex glasses of telescopes, 
pressing their convex sides updn one an- 
other, he observed several series of circles 
or rings of such colonrs, different, and of 
various intensities,' according to their dis- 
tance from the common central pellucid 
jrointof contact. 
As the colours vvere thus found to vary 
according to the different distances be- 
tween the glass plates, Sir Isaac conceived 
that they proceeded from the different 
thickness of the plate of air intercepted be- 
tween the glasses ; this plate of air being, by' 
the mere circumstance of thinness or thick- 
ness, disposed to reflect or transmit tlie rays 
of this or that particular colour. Hence, 
therefore, he concluded, that the colours 
of all natural bodies depend on their den- 
sity, or the magnitude of their component 
particles : and hence also he constructed a 
table, in which the thickness of a plate ne- 
cessary to reflect any particular colour was 
expressed in the millionth parts of an inch. 
From a great variety of such experi- 
ments, and observations upon them, our 
author deduced his theory of colours. And 
hence it seems that every substance in na- 
ture is transparent, provided it be made 
sufficiently thin ; as gold, the densest sub- 
stance we know of, when reduced into thin 
leaves, transmits abliiish gieen light tlirough 
it; If we suppose any body, therefore, as 
