L I G 
Ere the third dawning light 
Return, the ftars of morn fhall fee him rife 
Out of his grave, frefh as the dawning light. 
Milton. 
Life.—Infants that never faw light. Job. 
Swift roll the years, and rife the expefted morn. 
O fpring to light, aufpicious babe, be born! 
Pope. 
Artificial illumination.—Seven lamps fhall give light. Numb. 
■—Illumination of mind; inftruflion; knowledge: 
I will place within them as a guide 
My. umpire confcience, whom if they will hear, 
Light after light well us’d they fhall attain, 
And to the "end perfifling fafe arrive. Milton. 
The ordinary words of language, and our common ufe of 
them, would have given us light into the nature of our 
ideas, if confidered with attention. Locke. —The books of 
Varro concerning navigation are loft, which no doubt 
■would have given us great light in thofe matters. Arbuth- 
not on Coins. —The part of a picture which is drawn with 
bright colours, or in which the light is fuppofed to fall.— 
Never admit two equal lights in the fame pifture; but the 
greater light muft ftrike forcibly on thofe places of the 
pifture where the principal figures are; diminifhing as it 
comes nearer the borders. Dryden's Dufrefnoy. —Reach of 
knowledge; mental view.— Light, and underftanding, and 
wifdom, like the wifdom of the gods, was found in him. 
Daniel, v. xi.—Point of view; fituation; direction in which 
the light falls.—Frequent confideration of a thing wears 
off the ftrangenefs of it; and fliows it in its feveral lights, 
and various ways of appearance, to the view of the mind. 
South. —Public view; public notice: 
Why am I afk’d what next fhall fee the light f 
Heav’ns! was I born for nothing but to write? Pope. 
The public: 
Grave epiftles bring vice to light, 
Such as a king might read, a bifhop write. Pope. 
Explanation.—We fhould compare places of fcripture 
treating of the fame point; thus one part of the facred 
text could not fail to give light unto another. Locke's EJfay 
on St. Paul's Epijlles. —Any thing that gives light; a pha¬ 
ros; a taper; any luminous body.—Then he called for a 
Light, and fprang in and fell down before Paul. Aids, xvi. 
£9.—I put as great difference between our new lights and 
ancient truth, as between the fun and a meteor. Glanville. 
1 will make fome offers at their fafety, by fixing forne 
marks like lights upon a coaft, by which the fhips may 
avoid at leaft known rocks. Temple. 
That light you fee is burning in my hall; 
How far that little candle throws his beams! 
So fhines a good deed in a naughty world. Shahefpeare. 
Light, in the common acceptation of the word, figni- 
fies that invifible etherial matter which makes objects per¬ 
ceptible to our fenfe of feeing. Hence the 1110ft diftin- 
guifliing property of light is that by which it renders ob- 
jefts viiible, by fome power which transfers their exterior 
figure to the retina of the eye. 
We obtain light from three diftinft fources, namely, 
folar light, phofphorefcent light, and light of combuftion. 
I. Very little was known of the nature and properties 
of light before the experimental refearches of Newton ; 
and it is remarkable that at this time fo little fhould have 
been added to the labours of that acute philofopher. It 
is ftrange, that after the evidence of his experiments there 
could have been two opinions refpefting the nature of 
light. Huygens fuppofed the phenomena of light to be 
caufed by an undulatory motion, excited in a fuppofed 
fubtile and elaftic medium, pervading all fpace: thatthefe 
waves or pulfes are propagated, firft at the luminous body, 
fuch as the fun or a candle, and tranfmitted in all direc¬ 
tions. The imprefiion made by thefe waves upon the eye 
is the caufe of vifion. This doftrine has fince been taken 
wp by Euler, who, with much zeal and mathematical la- 
VOL. XII. No. 862. 
H T. 
hour, fupported it to his death. Newton, however, had 
given an hypothefis, fupported by clear and linking ex¬ 
periments; and at the fame time had pointed out infur- 
mountable objeftions to the undulatory hypothefis, fo 
that the labour and great talents of Euler were exhaulled 
to little purpofe. Sir Ifaac Newton argued, with great 
propriety, that the undulatory motion was inconfiftent 
with the phenomena of light. The paffage of light would 
not be confined to Itraight lines, but might, like found, be 
conveyed through crooked tubes; which is contrary to 
the faft. Befides, if light confifted in a mere predion, or 
pulfe, it would be propagated to all diftances in the fame 
inftant of time; the contrary of which appears from the 
phenomena of the eclipfes of Jupiter’s fateliites, as noticed 
under the article Astronomy, vol. ii. p. 386. 
We fhall therefore, with Newton, coniider light as a 
material agent, moving with an immenfe velocity from 
the point where it is liberated. If its motion be in free 
fpace, it moves in ftraight lines in the form of radii; and 
would continue for ever in the fame direction, if not 
changed in its courfe by the attraftion of other matter. 
Light, therefore, like ele&ricity and caloric, appears in a 
high degree to be repellent of itfelf, although it poflcffes 
attraftion for ponderable matter. Indeed, it is to this 
great repulfion of the particles of light that we are to at¬ 
tribute its progreifive velocity, which, therefore, is as the 
force by which its particles are feparated. 
The linall extent of the limits of vifion upon.the fur- 
face of our globe does not enable us to appreciate the 
velocity with which light moves. Roemer, aDanilh phi¬ 
lofopher, at length found the means of determining this 
point by the difference of time in the eclipfes of Jupiter’s 
fateliites, when the earth was on the fame or on the con¬ 
trary fide of the fun with that planet. The immerfions 
of thefe fateliites, as the Earth approaches towards Jupiter, 
are found to anticipate fomewhat on the true time, and 
to commence fooner; and again, as the Earth retires from 
Jupiter, their emerfions, which alone in that cafe can be 
obferved, happen later and later, or lofe time; deviating 
thus, very confiderably on either fide, from the true time 
marked by the tables. This was firft obferved by M. Roe¬ 
mer, and fince by other aftronomers; the reafon of which 
is not owing to any eccentricity; but apparently follows 
from this circumftance, that the light of the fun,- reflected 
from the fateliites, has farther to travel, before it reaches 
the eye, in the one cafe than in the other, by a fpace equal 
to the diameter of the earth’s annual orbit. The obfer- 
vations, whence this conclufion was deduced, were made 
at the obfervatory belonging to the Royal Academy of 
Sciences at Paris, from 1670 to 1675; the principal faft 
was, that the firlt fatellite fometimes emerged exadtly at 
the times calculated by the tables, and fometime not, in- 
fomuch that the greateft variation was about fourteen mi¬ 
nutes. . The particular obfervation that was the molt 
ftriking, was the emerfion of this fatellite obferved at Paris 
Nov. 9, 1676, ten minutes later than it had been obferved 
in the month of Auguft, when the Earth was much nearer 
to Jupiter. Hence Caffini and Roemer both concluded, 
that this circumftance depended on the diftance of Jupiter 
with refpeft to the earth; and that in order to account 
for it, they muft fuppofe that the light was about fourteen 
minutes in crofting the earth’s orbit. But the conclufion 
was afterwards abandoned and attacked by Monfieur Caf- 
fini. M. Roemer’s opinion found an able advocate in 
Dr. Halley; who removed Caffini’s difficulty, and left M. 
Roemer’s conclufion in its full force. In a memoir pre- 
fented to the academy in 1707, Monfieur Maraldi endea¬ 
voured to give a new force to Caffini’s arguments; but 
Roemer’s doflrine found a new defender in Mr. Pound. 
See Phil. Tranf. N° 136. 
It has been fince found, that, when the Earth is between 
the Sun and Jupiter, his fateliites are eclipfed about eight 
minutes fooner than they could be according to the tables; 
and that, when the Earth is nearly in the oppofite point of 
its orbit, thefe eclipfes happen about eight minutes later 
8 I than 
