It has since been found, by repeated experiments, that when the earth is ex actly between Jupiter and the sun, his satellites are seen eclipsed eight minutes and a quarter 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 ta bles predict them. Hence, then, it is cer tain that the motion of light is not instal), taneons, but that it takes up about six teen 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 hun dred 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 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 him till eight minutes and a quarter afterwards. Hence also, it is easy to know the time in which light travels to the earth, from the moon, or any of the other planets, or even from the fixed stars, when their dis tances, shall be known ; these distances are, however, so immensely great, that from the nearest of them, supposed to be Sirius, the dog-star, light takes up many years to travel to the earth : and it is even suspected, that there are many stars whose light has not yet arrived at us since their creation. And this, by-the-bye, may perhaps sometimes account for the ap pearance of new stars in the heavens. Our excellent astronomer, Dr. Bradley, afterwards found nearly the same velocity of light as Roemer, from his accurate ob servations, and most ingenious theory, to account for some apparent motions in the fixed stars ; for an account of which, see ABERRATION of light. By a long series of these observations, he found the dif, ference between the true and apparent place of several fixed stars, for different times of the year; which difference could no otherwise be accounted for, than for the progressive rays of light. From the mean quantity of this difference he ingeniously found, that the ratio the velocity of light to the velocity of the earth in its orbit, was as 10,313 to 1, or that light moves 10,313 times fister than the earth moves in its orbit about the sun ; and as this latter motion is at the rate of 18.1.4. miles per second nearly, it
follows that the former, or the velocity of light, is at the rate of about 195,000 miles in a second; a motion, according to which it will require just 8' 7" to move from the sun to the earth, or about 95,000,000 of miles.
It was also inferred, from the foregoing principles, that light proceeds with the same velocity from all the stsrs. And hence it follows, if we suppose that all the stars are not equally distant from us, as many arguments prove, that the motion of light, all the way it passes through the immense space above our atmosphere, is equable or uniform. And since the differ ent methods of determining the velocity of light thus agree in the result, it is rea sonable to conclude, that in the same medium, light is propagated with the same velocity after it has been reflected as before. For an account of Mr. Mel ville's hypothesis of the different velo cities of differently coloured rays, see COLOUR.
To the doctrine concerning the mate riality of light, and its amazing velocity, several objections have been made, of which the most considerable is ; that as rays of light are continually passing in different directions from every visible point, they must necessarily interfere with each other in such a manner as en tirely to confound all distinct perception of objects, if not quite to destroy the whole sense of seeing ; not to mention the continual waste of substance, which a constant emission of particles must oc casion in the luminous body, and there by, since the creation must have greatly diminished the matter iu the sun and stars, as well as increased the bulk of the earth and planets, by the vast quantity of particles of light absorbed by them in so long a. period of time. But it has been replied, that if light were not a body, but consisted in mere pression or pulsion, it could never be propagated in right lines, but would be continually inflected ad umbram. Thus, Sir Isaac Newton: " A pressure on a fluid medium, i. e. a motion propagated by such a medium, beyond any obstacle, which impedes any part of its motion, cannot be propagated in right lines, but will be always inflect ing and diffusing itself every way, to the quiescent medium beyond that obsta cle.