CHROMATICS. 
528 
obliquity of their poiition. It is highly probable that it 
is the fame coloured light, which being thrown, by the 
refraction of the atmofphere, into the fhadow of the earth, 
fometimes gives the moon, in a total eclipfe, the obfcure 
reddilh colour of brick. For the fame reafon, the colour 
of the moon will vary in eclipfes, according to the extent 
of atmofphere through which the rays have to traverfe. 
The doftrine of colours has been lately much improved 
by Mr. Delaval, as fated in a paper-communicated by 
him to the Literary and Philol'ophical Society of Man- 
chefter, and publifhed in their fecond volume of memoirs. 
He was led to this fubjeri, from a perfuafion of its utility 
to thofe interefting and elegant arts, whole object is the 
preparation and life of colouring fubftances r'juftly ob- 
ierving, that our views of experimental philofophy ihould 
not be confiiied to theory alone, but direfled alfo to its 
practical application. For, in proportion as the princi¬ 
ples of any fcience are unknown or mifconceived, the 
advancement of the arts, and manufactures which depend 
on them, mult, of Courfe, be impeded ; for, without thofe 
guides, neither much addition, nor any improvement, is 
to be expected. But when fcientilic principles are dif- 
clofed to the artift, he is enabled to draw from thofe ori¬ 
ginal fources an ample fore of ufeful inventions, by 
which his art is enriched ; and thus, the fpeculative 
fciences, by their extenfion to practical purpoles, become 
objeCts of great public importance. 
The arts of colour-making and dying were, in remote 
ages, carried to the height of perfection, in the countries 
of Phoenicia, Egypt, Paleltine, India, &c. The inhabi¬ 
tants of thofe countries excelled alfo in the art of imi¬ 
tating gems, and tinging glals and enamel of various co¬ 
lours. The colours ufed in very ancient paintings, were 
as various as thofe now in ufe, and greatly l'uperior both 
in beauty and durability. The paints ufed by Apelles 
were lb bright, that he was obliged to glaze his pictures 
with a dark-coloured varnilh, left the eye Ihould be of¬ 
fended by their brightnefs ; and even thofe were inferior 
to what had been uled among the ancient Egyptians. 
Notwitbftanding this perfection in dying and colouring, 
we find the Grecians and Romans continually degrading 
the ufeful arts. We may confider this as one of the molt 
fh'iking characters that diltinguilh the philofophy of the 
ancients from that of the moderns. The ancients being 
chiefly engaged in fpeculations that might procure them 
refpeCt, and attraCt applaufe, thought the ufeful arts un¬ 
worthy their attention ; whereas the moderns have culti¬ 
vated and promoted the ufeful arts ; and hence the Aca¬ 
demy of Sciences at Paris attempted to fhed the light of 
Icience upon the arts, by publilhing a defeription of them, 
grounded on the elevated idea, that the indultry of a na¬ 
tion cannot fail to be enlightened and increafed by a free 
communication of all the proceffes it ufes ; and that the 
facrifices it makes by this publicity, will ever be amply 
compenfated by the advantages it procures. 
The changes of colour in permanently coloured bodies, 
are produced by the fame laws which take place in tranf¬ 
parent colourlefs fubftances; and the experiments by 
which they can be inveftigated, confift of various methods 
of uniting the colouring particles into larger, or dividing 
them into linaller, maffes. The great Newton, as we have 
feen, made his experiments chiefly on tranfparent fub¬ 
ftances ; but, where he treats of others, he acknowledges 
his deficiency of experiments. He makes the following re¬ 
mark on thofe bodies which refleft one kind of light, and 
tranfmit another; viz. “that if thefe glafles or liquors 
were fo thick and maffy, that no light could get through 
them, he queftions whether they would not, like other 
opake bodies, appear of one and the fame colour, in all 
politions of the eye, though lie could not yet affirm it 
from experience.” It was an opinion of this great philo- 
fopher, that all coloured matter reflects the rays of light; 
fome reflefting copioufly the more, others the lefs, refran¬ 
gible rays. He was likewife of opinion, that opake bo¬ 
dies reflect the light from their anterior furface, by lome 
.s 
power of the body, evenly diffufed over, and external 
it. With refpedt to tranfparent coloured liquors, he fays, 
that a tranfparent body, which looks of any colour by 
tranfmitted light, may alio look of the fame colour by 
reflected light, the light of that colour being refledted by 
the farther furface of that body, or by the air beyond it"; 
and then the refledted colour will be diminilhed, and per¬ 
haps ceale, by making the body very thick, and pitching 
it on the back fide, to diminifh the refledtions of its far¬ 
ther furface, fo that the light reflected from the tinging 
particles may predominate. In fucli cafe the reflected light 
will be apt to vary from that which was tranfmitted. 
In order to inveftigate the truth of thefe opinions, Mr. 
Delaval entered upon a courfe of experiments with tranf¬ 
parent coloured liquors and glafles, as well as with opake 
•and femi-tranfparent fubftances. From thefe he found, 
that in tranfparent coloured fubftances, the colouring- 
matter does not refled any light, and when, by intercept¬ 
ing the light which vvas tranlinitted, it is hindered from 
pafling through fuch fubftances, they do not vary from 
their former colour to any other, but become entirely 
black. This incapacity of the colouring particles of tranf¬ 
parent bodies to refledl light, being deduced from very 
numerous experiments, it may be confidered as a general 
law. It appears the more extenfive, if we confider that, 
for the moll part, the tinging particles of tranfparent fub¬ 
ftances are extra-died from opake bodies; that the opake 
bodies owe their colour to thefe particles, as well as the 
tranfparent; and that by the lols of them they are de¬ 
prived of their colours. 
For his experiments Mr. Delaval ufed fmall phials ct 
flint-glais, calculated for the purpofe ; the form, that of 
a parallelepiped ; the height, exclufive of the neck, about 
two inches; the bale about an inch fquare, the neck two 
inches long. The bottom and three lides of each of thefe 
phials was covered with a black varnilh ; the cylindrical 
neck, and the anterior tide, except at the edges, being 
left uncovered. He was careful to avoid any crevices in 
the varnilh, that no light might be admitted, except 
through the neck or anterior fide of the phials. The phials 
Ihould be perfedlly clean, and thole liquors that depolit a 
fediment Ihould not be put into them, but at the time 
when the experiments are to be made. The uncovered 
fide likewife Ihould not be placed oppofite to the window 
where the light is admitted, becaule in that fituation the 
light would be refleded from the farthelt fide of the phial; 
lmooth black fubftances, refleding light powerfully, are 
bell fituated when the uncovered fide forms a right angle 
with the window. Having taken thefe precautions, he 
viewed a great number of lolutions, both of coloured me¬ 
tallic fait, and of the tinging matter of vegetables, ob- 
ferving that the colour by refledion was black, whatever 
it might be when viewed by tranfmitted light. If thefe 
colours are, however, fpread thin upon a white ground, 
they appear of the fame colour as when viewed by tranf¬ 
mitted light; but on a black ground they afford no co¬ 
lour, unlefs the black body be poiiflied, in which cafe 
the refledion of light through it produces the fame effed 
as tranlmilfion. 
The experiments made with coloured glafles were, in 
many refpeds, analogous to thofe with tranfparent co¬ 
loured liquors. For thefe he made feveral parcels of co¬ 
lourlels glals, compofed of borax and white fand. The 
glals was reduced to powder, and afterwards ground to¬ 
gether with the ingredients, by which the colour was to 
be imparted; a method he found preferable to the ul’ual 
mode of tinging glafles, as they became little inferior in 
lullre to real gems. The refulr of all his experiments was, 
that when matter is of fuch thinnefs, and the tinge fo di¬ 
lute, that light can be tranfmitted through it, the glafles 
then appear vividly coloured ; but when they are in large 
maffes, and the tinging matter is more denfely diffufed 
through them, they appear black ; for thefe, as well as 
the tranfparent liquors, Ihew their colour only by tranfi- 
jnilfion. Having in tins manner formed pieces of Inch 
glafs. 
