LIGHT. 
eye ; wliile the effect was ascribed by Py- 
thafronis solely to the particles proceeding 
from the external objects, and entering the 
pupil of the eye. But Aristotle defines 
light to be the act of a transparent body, 
considered as such ; and he observes, that 
light is not fire, nor yet any matter radiat- 
ing from the luminous 'body, and transmit- 
ted through the transparent one. 
The Cartesians have refined considerably 
on this notion ; and hold that light, as it ex- 
ists in the luminous body, is only a power 
or faculty of exciting in us a very clear and 
vivid sensation ; or that it is an invisible 
fluid present at all times and in all places, 
but requiring to be set in motion by a body 
ignited or otherwise properly qualified to 
make objects visible to us. 
Father Malbranche explains the nature 
of light from a supposed analogy between 
it and sound. Thus he supposes all the 
parts of a luminous body are in a rapid mo- 
tion, which, by very quick pulses, is con- 
stantly compressing the subtle matter be- 
tween the luminous body and the eye, and 
excites vibrations of pression : as these vi- 
brations are greater, the body appears 
more luminous ; and as they are quicker or 
slower, the body is of this or that colour. 
The Newtonians maintain, that light is not 
a fluid, but consists of a great number of 
very small particles, thrown off from the lu- 
minous body by a repulsive power, with an 
immense velocity, and in all directions. 
And these particles, it is also held, are emit- 
ted in right lines : which rectilinear motion 
they preserve till they are turned out of 
their path by some of the following causes, 
VIZ. bv the attraction of some other body 
near which they pass, which is called inflec- 
tion, or by passing obliquely through a me- 
dium of different density, which is called 
refraction ; or by being turned aside by the 
opposition of some intervening body, which 
is called reflection ; or lastly, by being to- 
tally stopped by some substance into which 
they penetrate, and which is called their 
extinction. A succession of these particles 
following one another, in an exact straight 
line, is called a ray of light ; and this ray, 
in whatever manner its direction may be 
changed, whether by refraction, reflection, 
or inflection, always preserves a rectilinear 
course, till it be again changed ; neither is 
it ])ossible to make it move in the arch of a 
circle, ellipsis, or other curve. For the 
above properties of the rays of light, see 
the several words Refraction, Reflec- 
tion, &c. 
Tlie velocity of the rays of light is truly- 
astonishing, amounting to nearly two hun- 
dred thousand miles in asecond oftime,which 
is about a million times greater than the ve- 
locity of a cannon ball. And this amazing ^ 
motion of light has been manifested in va- 
rious way.s, and first from the eclipses of Ju- 
piter’s satellites. It was first observed by 
Roemer, that the eclipses of those satellites 
happen sometimes sooner, and sometimes 
later, than the times given by the tables of 
them ; and that the observation was before 
or after the computed time, according as 
the earth was nearer to, or further from Ju- 
piter, than the mean distance. Hence Roe- 
mer and Cassini both concluded, that this cir- 
cumstance depended on the distance of Ju- 
piter from the earth ; and that, to account 
for it, they must suppose that the light was 
about fourteen minutes in crossing the 
earth’s orbit. This conclusion, however, 
was afterwards abandoned, and attacked 
by Cassini himself : but Roemer’s opinion 
found an able advocate in Dr. Halley,, who 
removed Cassini’s difficulty, and left Roe- 
mer’s conclusion in its full force. 
It has since been found, by repeated ex- 
periments, that when the earth is exactly 
between Jupiter and the sun, his satellites 
are seen eclipsed eight minutes and a quar- 
ter sooner than they could be according to 
the tables ; but when the earth is nearly in 
the opposite point of its orbit, these eclipses 
happen about eight minutes and a quarter 
later than the tables predict them. Hence, 
then, it is certain that the motion of light is 
not instantaneous, but that it takes up 
about sixteen minutes and a half of time to 
pass over a space equal to the diameter of 
the earth’s orbit, which is at least one hun- 
dred and ninety millions of miles in length, 
or at the rate of near two hundred thousand 
miles per second, as above-mentioned. 
Hence, therefore, light takes up about 
eight minutes and a quarter in passing from 
the sun to the earth ; so that, if he should 
be annihilated, we should see him for eight 
minutes and a quarter’ after that event 
should happen ; and if he were again created, 
we should not see hirn till eight minutes 
and a quarter afterwards. Hence also it is 
easy to know the time in which light tra- 
vels to the earth, from the moon, or any of 
the other planets, or even from the fixed 
stars, when their distances shall be known ; 
these distances are, however, so immensely 
great, that from the nearest of them, sup- 
posed to be Sirius, the dog:Star, light takes 
up many years to travel to the earth : and 
