CHROMATICS, 527 
-fakes fepar&tely, to tie, In fome tfleafure, tranfparent: and 
the opacity arilcs from the multitude of reflections caufed 
in their internal parts. This obfervation will be ealily 
granted by thofe who have been converlant with micro- 
fcopes; for there they are found to be, for the moll part, 
tranfparent. Nothing feems more opake, and free from 
tranfparency, than the clothes we wear. Yet let us only 
examine one of the woollen hairs that enter into its com- 
pofition, with a microfcope, and we (hall find it to be 
nearly tranfparent. Gold in the mafs lets no light pals 
through it l but if beaten out extremely thin, we (hall 
then fee that its parts are tranfparent, like other bodies. 
If held over a hole, in a darkened window, it will appear 
of a greenifli hue. If gold be compofed of tranfparent 
parts, we may furely conclude the fame of other bodies ; 
and, indeed, very few are to be found, in which, if re¬ 
duced to fufficient thinuefs, and applied to a hole through 
which the fun’s rays pafs, a degree of tranfparency is not 
inanifelt. 
It now becomes Receffary, fmce light finds a free paf- 
fage through the leafl particles, to inquire what tenders 
them Opake ; and this, by fir Ifaac Newton, is attributed 
to the multitude of reflection's and refractions which take 
place in its interior parts; there being, between the parts 
of opake or coloured bodies, a number of fpaces, filled 
with mediums of a different denfity from that of the body, 
as water between the tinging corpufcles with which any 
liquor is impregnated ; air between the aqueous globules 
that conftitute clouds and milts, &c. Thele fpaces cannot 
be traverfed by light, without refraCling or reflecting it 
in various ways, by which it is prevented from palling on 
in a ftraight line, which it would do if the parts were 
continuous, without any fuch interllices between them ; 
for we have already learned, that reflections are only made 
at the fuperficies of mediums-of different denfities. The 
opacity of a body ariles, therefore, from the difeontinuity 
of its particles, and the different denfity of the interve¬ 
ning mediums and the particles which compofe them. 
This idea of opacity is greatly confirmed, by confidering 
that opake bodies become tranfparent by filling \»p the 
pores with any fubltance of nearly the fame denfity with 
their parts. Thus when paper is wet with oil or water, 
or when linen cloth is dipped in water, oiled, or var- 
nilhed, or the oculus mundi lleeped in wafer, &c. they 
become more tranfparent than they were before; as filling 
the pores of an opake body makes it tranfparent, fo, on 
the other hand, evacuating the pores of a tranfparent bo¬ 
dy, or feparating its parts, renders it opake; as falts, or 
wet paper, by being dried ; horn, by being lcraped; glafs, 
by being reduced to powder, or otherwife flawed ; tur¬ 
pentine, by being ftirred about with water, till they mix 
imperfectly; and water, by being formed into many fmall 
bubbles, either in the form of froth, or, by (baking it 
together with oil of turpentine, or fome other convenient 
liquor, with which it will not combi.ne. It is plain, there¬ 
fore, that it is in homogeneity we are to feek for the catife 
of tranfparency. If there be many pores in a bodyq and 
thefe be filled with a matter differing much in denfity from 
the body itfelf, the light will meet with a thoufand refrac¬ 
tions and reflections in the internal parts, and will thus 
be utterly extinguiftred. But the parts of bodies, and their 
interltices, mult not be lefs than fome definite fize, to be¬ 
come opake and coloured. For the moll opake bodies, if 
their parts be fufficiently divided, as metals, by being dif- 
folved in acid menllrua, become perfectly tranfparent. 
The black fpot, near the point of contaCl of the two plates 
of glafs, it has been obferved, tranfmitted the whole light 
w here the glades did not abfolutely touch; and the re¬ 
flection at the thinned part of the l'oap-bubble was fo in- 
ienfible as to make that part appear intenfely black, by 
the want of reflected light. It is oh thefe grounds, that 
water, fait, glafs, (tones, &c. are tranfparent; for,-from 
many confiderations, they feem to be as full of pores as 
other bodies are, yet their particles- and pores are too 
imall to cauie reflection in their common furfaces. 
The tranfparent parts of bodies, according to their fg- 
veral fizes, muff refleCt rays of one colour, and tranfmit 
thofe of others, on the fame principles that thin plates or 
bubbles do refleCt or tranfmit thefe rays; and this feems 
to be the ground of all their colours. That they do fo 
is plain from various obfervations ; and it is on thele prin« 
ciples that we explain the variety of colours feen in fome 
fiiks, on pigeons’ necks, peacocks’ tails, and the feathers 
of other finely-coloured birds. If the eye be fixed on a 
pigeon’s neck, and both be kept at reft, only one colour 
is ohfervable ; but if either moves, el'peciaily the latter, 
a different colour may be feen. Shady fiiks are woven 
with threads of different colours; one arranged longitu¬ 
dinally, the other tranfverfely ; and as the greater or lefs 
proportion of either of thefe appears, fo one or the other 
of the colours will prevail. Wet thefe double-coloured 
objects, dip the variegated featherin water, orthe change¬ 
able fiik in oil, their reflections will be lefs vivid, and they 
will return but one uniform fnade of colouring. The (kin 
of the cameiion is tranfparent, its ground being between a 
paje red and yellow, coloured with a number of fmall finootbr 
protuberances of a cold blueifti colour. It is endowed 
with a faculty of blowing up or contracting its (kin at 
will. This caufes the different colours, in appearance, to 
vary ; it therefore fometimes appears reddifli, at others 
blue: the j'ellow rays of the (kin, occafionalty mixing 
with the blue of the protuberances, produce the idea of 
green ; and w'hen placed on a red or yellow lubftance, its 
natural colours are unavoidably heightened. 
From various phenomena it is evident, that a great pro¬ 
portion of the fainter coloured rays are (topped in the«r 
paflage through the atmofphere, and are thence reflected 
upon other bodies; while the red and orange rays are 
tranfmjtted to greater diftances. This circumftance ex¬ 
plains the blue (hadows of bodies, the blue colour of the 
(ky, and the red colour of the clouds, when the fun is 
near the horizon. At particular times, when the (ky is 
plear and ferene, in the morning and the evening, the 
(hadows caft from opake bodies have been obferved to be 
tinged with blue and green. This circumftance naturally 
refults from the minute particles of the. atmofphere re¬ 
flecting the deiicate and moft refrangible rays, the blue 
and violet, for inliance, which occaiions a predominance 
of thefe hues. 
The bluenefs of the (ky is accounted .for on the fame 
principles; namely, the copious reflection of the blue rays 
by the atmofphere, which produces the effeCl of an arch 
of that colour all around us. This is occafionally diverfir 
fled by the greater denfity of the vapours, which refleCt 
the (tronger rays. The coloured clouds, in particular, 
which appear towards the morning and evening, when the 
fun is in or near the horizon, are to be attributed to the 
fame caufe. Th 4 rays of light traverfing a vail extent of 
atmofphere ; the fainter and more deiicate rays, as the 
blue and violet, are detached by repeated reflections of 
the atmofpheric particles; and the (tronger rays, as the 
red, the orange, &c. are permitted to proceed, and reach 
the clouds, from whence they are reflected. Agreeable- 
to this theory, we may obferve, that the fun’s horizontal 
light is fometimes fo deeply tinClured with the red, that 
objects illuminated, by it frequently appear of a bright 
orange, and even red. It is obfervable, that the clouds 
do not, in common, affume their brighter dyes till the 
fun is fome minutes let, and that they pafs from yellow 
to a flaming gold colour; and thence, by degrees, to red, 
which becomes deeper and deeper, till at length the dif— 
appearance of the fun leaves them of a leaden hue, by the 
reflection of the blue light from the air. A (imilar change 
of colour is obferved on the (howy tops of the Alps ; and 
the fame may be feen, though lefs ftrongly, on the eaftem 
and weftern fronts of white buildings.. St. Paul’s church, 
in London, is a good object of this kind, and is often, at 
fun-fet, tinged with a confiderable degree of redneis. 
What makes the fame colours more rich and copious in 
the clouds, is their femi-1ranfparepcv, joined with the 
, ‘ ' obliquity- 
