07S LIGHT. 
than the tables predict them. Hence it is undeniably cer¬ 
tain, that the motion of light is not indantaneous, fmcc 
it takes about i6£ minutes of time to go through a fpace 
equal to the diameter of the earth’s orbit, which is at leaft 
190 millions of miles in length; and confequently the 
particles of light fly about 191,919 or 200,000 miles every 
i'econd of time, which is near a million of miles fwifter 
than the motion of a cannon-ball. And, as light is i6f mi¬ 
nutes in travelling acrols the Earth’s orbit, it mud be 8J 
minutes in coming from the Sun to us; therefore, if the 
fun were annihilated, we fhould fee him for 8J- minutes 
after; and, if he were again created, he would be 8^ mi¬ 
nutes old before we could fee him. 
The firft perfon, who conceived the thought of rneafur- 
ing the velocity of light, was Galileo, who has given a 
particular defcription of his contrivance for this purpofe, 
in his Treatife on Mechanics, p. 39. He had two men 
with lights, one of whom was to obferve when the other 
uncovered his light, and to exhibit his own the moment 
that he perceived it; the experiment was tried, as may 
naturally be imagined, without fuccefs, at the didance of 
one mile ; but the members of the Academy del Cimento 
refumed the experiment, and placed the obfervers, to as 
little purpofe, at the didance of two miles. However, the 
method ufed by M. Roerner, already mentioned, was the 
only one adequate to the difcovery of the velocity of light. 
See the article Optics. 
The wonderful divisibility of the parts of matter is no¬ 
where more apparent than in the minutenefs of the parti¬ 
cles of light. Dr. Nieuwentyt has computed, that an inch 
of candle, when converted to light, becomes divided into 
269,617,04.0 parts, with 40 ciphers annexed ; at which 
rate there mud ilfue out of it, when burning, 418,660, 
with 39 ciphers more, particles in the fecond of a minute; 
vaflly more than a tlioufand times a thoufand million 
times the number of fands the whole earth can contain ; 
reckoning ten inches to one foot, and that 100 fands are 
equal to one inch. See Relig. Philof. vol. iii. p. 865. 
It mud be acknowledged, that many difficulties and ob¬ 
jections have been urged againd the materiality of light, 
or the hypothefis of light’s confiding of fmall particles 
emitted from luminous bodies; and that many eminent 
philofophers, both foreigners and Englifli, have recurred 
to the opinion, that light confids of vibrations propagated 
from the luminous body through a fubtile e.therial medium. 
The ingenious Dr. Franklin, in a letter dated April 23d, 
1752, expreffes his diflatisfaftion with the doCtrine, that 
fuppofes particles of matter, called light, continually driven 
od' from the fun’s furface, with a fwiftnefs fo prodigious. 
“Mud not,” fays he, “the fmalleft portion conceivable 
have, with fuch a motion, a force exceeding that of a 
twenty-four pounder dilcharged from a cannon ? Mud 
not the fun diminifh exceedingly by fuch a wade of mat¬ 
ter; and the planets, indead of drawing nearer to him, as 
fome have feared, recede to greater diftances through the 
Jeffened attraction ? Yet thefe particles, with this amazing 
motion, will not drive before them, or remove, the lead 
and lighted dud they meet with; and the fun appears to 
continue of his ancient dimenlions, and his attendants 
move in their ancient orbits.” Accordingly, he conjec¬ 
tures, that all the phenomena of light may be more con¬ 
veniently folved, by fuppofing univerfnl fpace filled with 
a fubtile eladic fluid ; which, when at red, is not vifible, 
but whofe vibrations affeCl that fine fenfe in the eye, as 
thofe of air do the groffer organs of the ear; and that dif¬ 
ferent degrees of the vibration of this medium may occa- 
iion the appearances of different colours. The eladic 
fluid, he fays, is always the fame, and yet weaker and 
ftronger fparks differ in apparent colour, fome white, blue, 
purple, red ; the dronged, white; the weak ones, red. 
Dr. Horfley has taken confiderable pains to obviate the 
difficulties fuggeded by Dr. Franklin; and fuppofing that 
the diameter of each particle of light does not exceed one 
millionth of one millionth of an inch, and that the den¬ 
sity of each particle is three times that of iron, that the 
light of the fun traverfes the femi-diameter of the orbia 
magnus in 7', and that this femi-diametcr is 22919 femi- 
diameters of the earth, he calculates, that the momentum 
or force of motion in each particle of light coming from 
the fun is lefs than that in an iron ball of |th of an inch 
diameter, moving at the rate of lefs than an inch in twelve 
thoufand millions of millions of Egyptian years. Hence, 
he concludes, that a particle of matter, which is probably 
larger than any particle of light, moving with the velocity 
of light, has a force of motion, which, indead of exceed¬ 
ing the force of a twenty-four pounder difcharged from a 
cannon, is infinitely lefs than that of the fmalleft (hot dif¬ 
charged from a pocket-piftol, or lefs than any that art can 
create. Moreover, he thinks it poffible, that light may be 
produced by a continual emiffion of matter from the fun, 
without any fuch wade of his fubdance as fliould fenfibly 
contract his dimenlions, or alter the motions of the pla¬ 
nets, within any moderate length of time. In proof of 
this, he obferves, that it is not neceflary to the production 
of any of the phenomena of light, that the emanation 
from the fun Ihould be continual in a ftrict mathematical 
fenfe, or without any interval; and likewife that part of 
the light which ilTues from the fun is continual!, return¬ 
ing to him by reflection from the planets, and other light 
is continually coming to him from the funs of other lyf- 
tems. He proceeds by calculation to fliow, that in 
385,130,000 Egyptian years, the fun would lofe of 
his matter ; and therefore that the gravitation towards the 
fun, at any given didance, would diminilh in the fame pro¬ 
portion. But this alteration is much too fmall to dicover- 
itfelf in the motion of the earth, or of any of the planets.. 
He alfo computes, that the greated droke which the retina 
of a common eye fudains, when the eye, in a bright day,., 
is turned up direCtly to the fun, does not exceed that 
which an iron fliot, £th of an inch diameter, would give, 
moving only at the rate of i6'i6 inches in a year; but 
the ordinary droke is lefs than the ^-Jg-^th part of this. 
One of the principal difficulties attending the hypothefis 
of the materiality of light, is the non-interference of its 
particles with each other. There is, probably, lays Mr. 
Melville, Edinb. Eff. vol.ii. p. 17, &rc. no phylical poinE 
in the vifible horizon, that does not fend rays to every 
other point, unlefs where opaque bodies interpole. Light, 
in its paffage from one fyftem to another, often paffes 
through torrents of light ifluing from other funs and fyf- 
tems, without ever interfering, or being diverted from its 
courfe either by it, or by the particles of that eladic me¬ 
dium, which fome have fuppofed to be diffufed through 
all the mundane fpace. In accounting for this faCt, he 
fuppofes that the particles of light mud be incomparably 
rare, even when they are the mod denfe ; that is, that the 
femi-diameters of two of the neared particles, in the fame 
or in different beams, foon after their emifiion, are incom¬ 
parably lefs than their didance from one another. This 
conlideration obviates the objection urged by Euler and 
others againd the materiality of light, from its influence 
in didurbing the freedom and perpetuity of the celedial 
motions. The difficulty attending the fuppofition, that 
particles of light move through other light, in all imagi¬ 
nable directions, without perpetual collifions among the 
particles, and continual deflections from a rectilinear 
courfe, is, in a great degree, obviated, by an eafy compu¬ 
tation of Mr. Canton. He obferves, that it is neceffary 
to allow only a very fmall portion of time between the 
emiffion of every particle and the next that follows in 
the fame direction. Suppofe, for indance, that one lucid 
point of the fun’s furface emits 150 particles in one fe¬ 
cond, which are more than fufficient to give continual 
light to the eye, without the lead appearance of inter- 
miffion ; yet dill the particles of which it conlifts will, on 
account of their great velocity, be more than a thoufand 
miles behind one another, and thereby leave room enough 
for others to pafs in all directions, (Phil. Tranf. vol. Iviii. 
aft. 45.) But, if we adopt the concluiions drawn from 
d’Arcy’s experiments on the duration of the fenfations 
excited 
