CHROMATICS. 
SH 
CHROMA'TICS, or Chromatography, f. [from 
colour, and ygpttyu, to defcribe.] The fcience 
which inveftigates the natural colour of bodies ; and, 
when applied to light, cohftitutes a principal branch of 
optics. Before the time of lir Ifaac Newton, philofophers 
were formerly of opinion, that the folar light was Ample and 
uniform, without any difference or variety in its parts, 
and that the different colours of objects were made by re- 
fiadtion, reflection, orfliadows. But Newton taught them 
the errors of their former opinions; he fhewed them to 
difleCt a Angle ray of light with the minutelt precifion, 
and demonltrated that every ray was itfelf a compofition 
of leveral rays all of different colours, each of which when 
feparate held to its own nature, Ample and unchanged by 
every experiment that could be tried upon it. Or to be 
more particular, light is not all fimilar and homogeneal, 
but compounded of heterogeneal and diflimilar rays, fome 
of which in like instances being more refrangible, and 
others lefs refrangible, and thole which are molt refran¬ 
gible are alfo molt inflexible; and according as they 
differ in refrangibility and reflexibility, they are endowed 
with the power of producing or of exciting in us fenfa- 
tions of different colours. 
Sir lAiac Newton's theory of light and colours is link¬ 
ing and beautiful in itfelf, and deduced from clear and 
deciflve experiments, and may be almolt faid to demon- 
ftrafe clearly, i. That lights Which differ in colour, differ 
alfo in degrees of refrangibility. That the light of the 
fun, notwithffanding its uniform appearance, conflits of 
rays differently refrangible. 3. That thofe rays which are 
more refrangible than others, are alfo more reflexible. 
4. That as the rays of light differ in degrees of refrangi- 
bilitity and reflexibility, fo they alfo differ in their difpo- 
iition to exhibit this or that particular colour; and that 
colours are not qualiflcations of light derived from re¬ 
fractions or reflections of natural bodies, as was generally 
believed, but original and connate properties, which are 
different in different rays, fome rays being difpofed to 
exhibit a red colour and no other, and fome a green and 
no other, and fo of the reft of the prifmatic colours. 5. 
That the light of the fun coniifts of violet-making, indi¬ 
go-making blue-making, green-making, yellow-making, 
orage-making, and red-making, rays; and all of thele 
are different in their degrees of refrangibility and reflexi¬ 
bility ; for the rays which produce red colours are the 
leaft refrangible, and thofe that make the violet the moll; 
and the reft are more or lefs refrangible as they approach 
either of thele extremes, in the order already mentioned ; 
that is, orange is leaft refrangible next to red, yellow next 
to orange, and fo on; fo that, to the lame degree of re¬ 
frangibility, there ever belongs the fame colour, and to 
the fame colour the fame degree of refrangibility. 6. 
Every homogeneal ray, confldered apart, is refraCted ac¬ 
cording to one and the fame ruie, fo that its Ane of inci¬ 
dence is to its Ane of refraCiion in a given ratio; that is, 
every different coloured ray has a different ratio belong¬ 
ing to it. 7. The fpecies of colour, and degree of refran¬ 
gibility and reflexibility, proper to any particular fort of 
rays, is not mutable by reflection or refraction from natu¬ 
ral bodies, nor by any other caufe that has been yet ob~ 
ierved. When any one kind of ray has been feparated 
from thofe of other kinds, it has obltinately retained its 
colours, notwithftanding all endeavours to bring about 
a change. 8. Yet teeming tranfmutation of colours may 
be made, where there is any mixture of divers forts of 
rays; for, in luch mixtures, the component colours ap¬ 
pear not, but, by their mutually alloying each other, con- 
ftitute an intermediate colour. 9. There are, therefore, 
two iorts of colour, the one original and Ample, the other 
compounded of thefe; and all the colours in the univerfe 
are either the colours of homogeneal, Ample light, or 
compounded of thele mixed together in certain propor¬ 
tions. The colours of Ample light are, as we obferved 
before, violet, indigo, blue, green, yellow, orange, and 
red, together with an indefinite variety of intermediate 
gradations. The colours of compounded light are dif¬ 
ferently compounded of thefe Ample rays, mixed in vari- 
rious proportions : thus a mixture of yellow-making and 
blue-making rays exhibits a green colour, and a mixture 
of red and yellow makes an orange; and in any colour 
the fame in fpscie with the primary ones may be pro¬ 
duced by tire compofition of the two colours next ad¬ 
jacent in the feries of colours generated by the prilm, 
whereof the one is next molt refrangible, and the other 
next leaft refrangible. But this is not the cafe with thole 
which are Atuated at too great a diftance; orange and 
indigo do not produce the intermediate green, nor lcarlet 
and green the intermediate yellow. 10. The molt fur- 
priling and wonderful compofition of light, is that of 
whitenefs ; there is no one fort of rays which can alope 
exhibit that colour: it is ever compounded, and to its 
compofition all the aforefaid primary colours are requi- 
Ate. 11. As whitenefs is produced by a copious refledfion 
of rays of all forts of colours, when there is a due pro¬ 
portion in the mixture; fo, on the contrary, blackneis is 
produced by a fuffocation and abforption of the incident 
light, which being ftopped and fupprefled in the black 
body, is not refledted outward, but refledted and refradted 
within the body till it be ftifted and loft. 
The foundation of a rational theory being thus laid, it 
next became natural to enquire by what peculiar me- 
chanifm in the ftrudture of each particular body, it was 
fitted to refledt one kind of rays more than another. This 
fir Ifaac Newton attributes to the denfity of thele bodies. 
This lubjedt, however, is not fo clear as the preceding; 
for the prelent theory fuggefts many doubts to every in- 
quifitive mind, and is allowed by all to be attended with 
difficulties. There are no optical experiments, however, 
in which fir Ifaac Newton leems to have taken more pains, 
than thofe relating to the rings of colours which appear 
in thin plates, and which we now propofe to explain. In 
all his obfervations and inveftigations concerning them, 
he dilcovers the greateft fagacity, both as a philofopher 
and a mathematician. , 
The bubbles which children blow with a mixture of 
loap and water, were oblerved by Dr. Hooke to exhibit 
various colours according to their thinnefs, and that when 
they have a conliderable degree of tliicknefs they appear 
colourlefs ; from this the prefent theory has taken its rife. 
It is thus that things overlooked by the reft of mankind, 
are often the moll fertile in fuggelting hints to thofe who 
are habituated to refledtion. 
Sir Ifaac Newton blew up a large bubble from a ftrong 
mixture of foap and water, and let himfelf attentively to 
confider the different changes of colour it underwent, from 
its enlargement to its diffolution. He in general per¬ 
ceived that the thinner the plate of water which com- 
pofed the fides of the bubble, the more it reflected the 
violet-colour ray; and that in proportion as the fides of 
the bubble were more thick and denfe, the more they re¬ 
fledted the red ; he, therefore, was induced to believe, that 
the colours of all bodies proceeded from the tbicknels and 
denfity of the little tranfparent plates of which they are 
compoled. To bring this opinion nearer to certainty, it 
was neceffary to mealure the tliicknefs of the plate of water 
which compofed the bubble; but this was a matter of 
great difficulty, as the bubble was of itfelf of too tran- 
lient a nature to undergo the neceffary experiments. 
Our philofopher, ever fertile in expedients, recolledled 
having oblerved, that as two prifms were comprefl'ed hard 
together, in order to make their fides (which happened to 
be a little convex) touch one another, they were both as 
perfedtly tranfparent in the place of Contadf as if they had 
been but one piece of glafs; but that round the point of 
contadl, where the gtafles were a little feparated from each 
other, rings of different colours appeared. 
To obferve more accurately the order of the colours 
produced in this manner, he placed a glafs lens, whofe 
convexity was very fm'all, upon a plain glafs. New it is 
evident, that thole would only touch at one particular 
point; and, therefore, at all other places between the ad¬ 
jacent furfaces, a thin plate of air was interpofed, whole 
thicknels 
