C HR O M ATI C S. 
526 
obtained by lenfes. One of tlie mod remarkable circum- 
Itances attending this method of making the experiment. 
Is the facility with which the colours may be removed, or 
even made to difappear, by heats too low to feparate the 
glaffes. A touch of the finger immediately caufes the ir¬ 
regular rings of colours to contract towards their center, 
in the part touched. 
Thefe experiments render it evident, that the colours 
of bodies depend, in fome degree, upon the thicknefs and 
denfity of the particles that compofe them. Hence, if the 
denlity, or fize of the/particles, in the furface of a body, 
be changed, the colour is likewife changed. When the 
thickneis of the particles of a body is fuch, that one fort 
of light, or one fort of colour, is refledfed ; another light, 
or other colours, will be tranfmitted; and therefore the 
body will appear of the firlt colour. 
' There is a certain determinate thicknefs which fecms 
to be neceflary in a plate of water, for example, in order 
to refledl a particular colour,\and a different thicknefs to 
make it reflect any other colour; and in general, that 
a lefs thicknefs is neceflary, to reflect the moil refrangi¬ 
ble rays, as violet and indigo, than thofe which are lead 
refrangible, as the red and orange-coloured rays. The 
particles of bodies reflect rays of one colour, and tra-nf- 
mit thofe of another; and this is the ground of all their 
colours. 
Sir Ifaac Newton, in order to account for the intervals 
of the coloured rings in thefe thin plates, and alfo all 
other cafes of the refledtion or tranfmiflion of light, ad¬ 
vances an hypothecs; but, like a wife and cautipus phi- 
lofopher, he profeffes not to lay much ftrefs upon it, 
though he feems not to entertain any fufpicion of its 
being fallacious. Indeed, it feems to be a kind of fair 
inference from the experiments we have been defcribing. 
The hypothecs is this : that every ray of light is, at its 
fir ft emiffion from the luminous body, put into a tranfient 
ftate or conftitution, which, in its progrefs, returns at 
* equal intervals, difpofmg it, at every return, to be eafity 
tranfmitted into any refradting furface it may meet with ; 
whereas in the intervals between thefe returns, it is dif- 
pofed to be eafily reflected ; fo that, upon the arrival of a 
number of rays of light at the furface of every medium, 
thofe of them in which they were difpofed to be tranf- 
mitted eaiily, would pafs the -interval between the two 
mediums ; and thofe which were in a contrary ftate, would 
be refledfed; on which account, fome light is generally 
reflected, and fome tranfmitted, at every different furface 
on which it falls. Thofe ftates, into which the rays of 
light are put, he calls fits of eafy refledtion and tranf- 
miftion. This hypothefis, however, is not without diffi¬ 
culties, and mult, therefore, be received with caution, as 
it was propofed, till it (hall be either confirmed or con¬ 
futed by experiment, and a new theory fubftituted in its 
Head. When we are brought, as it were, to the cbnfines 
of material nature, -we mult expedt to meet with fome 
confuliou and darknefs in our explanations. There are 
barriers to our knowledge, which cannot be paffed by any 
force of human faculties. Sir Ifaac Newton, the legifla- 
tor of philofophers, exprefled, under the form of conjec¬ 
tures or queftions, thofe things which he was unable fa- 
tisf'adtorily to refolve; avoiding rafh affertions, which are 
fo fondly taken up by thofe who wifli to gain a momen¬ 
tary reputation. 
Newton conjedtured, that thefe fits of eafy refledtion 
and tninfmiffion may be occafioned by the vibrations of 
a fubtil fluid, in which the ray paiTes; any ray being dif¬ 
pofed to be tranfmitted when the vibration coincides with 
it, and to be refledted when it is thereby counteradted. 
He alfo thought that thefe vibrations might be excited 
by the mutual adtion and re-adfion of the light in bodies, 
and of this medium, at the inftant of refradtion and re- 
fiedlion. He therefore fuppofed two caufes of this difpo- 
fition to be refledted or tranfmitted, when rays of light 
arrive at any new furface. One of them is the regular 
vibration of the etherial medium, affediing them through 
the whole of their progrefs from the luminous body; and 
the other the tremulous motion, or irregular vibration of 
the fame medium, at the furfaces of bodies, occafioned 
by the adlion and re-adfion between thofe bodies and 
light. Thus, as ftones, by falling into water, put the 
water into an undulating motion ; and all bodies, by per- 
cufllon, excite vibrations in the air; fo the rayjsof light, 
by impinging on any refradling or refledting furface, ex¬ 
cite vibrations in the refradling or reflecting medium, and, 
by exciting thefe, agitate the lblid parts of the refradting- 
or refledfing body ; and that the vibrations tivus excited 
in this fubtil refracting or refledfing medium are propa¬ 
gated much after the manner that .vibrations are propa¬ 
gated in the air, caufing found, and moving falter than 
the rays, 1b as to overtake them ; and that when any ray 
is in that part of the vibration which confpires with its 
motion, it eafily breaks through a refradling furface ; but 
when it is in the contrary part of the vibration which 
impedes its motion, it is eafily reflected.; and, by confe- 
quence, that every ray is fucceffively difpofed to be eafily 
refledted, or eafily tranfmitted by every vibration by which 
it is overtaken. 
It has already been ftated, that the colours of natural 
bodies confift in a difpofition to refledt one fort of rays 
more copioufly than another; and that other bodies are 
of a different colour, becaufe they refledl rays of a differ¬ 
ent kind. So that if light confided only of one kind of 
rays, there could be only one colour in the world; nor 
would it be pofiible, by refradfions and refledtions, to 
produce a new one. Thus, in fome bodies, all the rays 
are extinguilhed but the red-making; and when they are 
refledfed to our eyes, they excite in us the idea of red ; 
and thence we fay, that fuch a piece of cloth, &c. is red 5 
attributing that only to the cloth or wood, which more 
particularly arifes from the light which dreffes them in 
their various beaut}'-. Thus the ruby abforbs the green, 
the blue, and the violet; but reflects the red-making rays 
to our eye, with all their prifmatic luftre. The amethyft 
imbibes the ftronger rays, and gives back the violet with 
milder brightnefs. The j'onquil gives us only yellow, 
and the hyacinth its vivid blue. Every coloured objedt 
may be thus regarded as a partial divider of the rays, fe- 
parating one or more colours, and confounding ail the 
others. Thofe furfaces of tranfparent bodies, which have 
the greateft refradling power, refledl the greateft quantity 
of light. In other words, bodies, by which the light is 
more refracted, do likewife more ltrongly refledl it. Dia¬ 
monds, which refradt the light very ftrongly, give it, in 
proportion, a ftronger refledtion: and hence proceed the 
vivacity of their colours, and their lparkling etfedl. 
We fhail perceive the analogy between refradtion and 
refledtion; by confide-ring that the moll refradtive medium 
totally refiedts the fays of light, at certain degrees of in¬ 
cidence. But the truth of the propofition further appears, 
by obferving the tranfparent bodies, fuch as air, water, 
oil, glafs. Ifland cryftal, white tranfparent arfenic, and 
diamond, have a ltronger or weaker refledtion, according 
to the greater or lefs refradtive powers of the mediums 
that are contiguous to them. Thus at the confine of air 
and fal gem, it is ftronger than at the confine of air and 
water; and ltill ftronger between common air and glafs ; 
Hill more fo between air and a diamond. If any of thefe 
be immerfed in water, its refledtion becomes weaker than 
before ; and it is weaker (till, if it be immerfed in liquors 
of a greater refradtive power. If water be divided into 
two parts, by any imaginary furface, there is no reflection 
at the confine of thole two parts ; and for the fame reafor, 
there can be no fenlible refledtion in the confine of the 
two glalfes of equal denlity. The reafon, therefore, why 
all pellucid mediums have no fenlible refledtion but at 
their external furfaces, where they are contiguous to me¬ 
diums of different denfities, is, that their contiguous parts 
have precifely the lame degree of denfity. 
The leaft parts of all bodies, though feemingly void .off 
tranfparency, when viewed in tli£ grols, will be found, if 
taken 
