LIGHT. 



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 Hoemer, 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 of 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 faster than 

 the earth moves in its orbit about the 

 sun ; and as this latter motion is at the 

 rate of 18.J.1. miles per second nearly, it 

 follows that the former, or the velocitv 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 stars. 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 

 COLOX:H. 



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 

 Y/ith 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 in 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. 



The power of gravity tends down- 

 wards; but the pressure of water arising 

 from it tends every way with an equable 

 force, and is propagated, with equal ease 

 and equal strength, in curves as in straight 

 lines. Waves, on the surface of the wa- 

 ter, gliding by the extremes of any ver\ 

 large obstacle, inflate and dilate them- 

 selves, still diffusing gradually into the 

 quiescent water beyond that obstacle. 

 The waves, pulses, or vibrations of the 

 air, wherein sound consists, are manifest- 

 ly inflected, though not so considerably 

 as the waves of water ; and sounds art 

 propagated with equal ease through, 

 crooked tubes and through straight lines , 

 but light was never known to move iu 

 any curve, nor to inflect itself ad urn- 

 dram. 1 ' 



It must be acknowledged, however, 

 that many philosophers, both English and 

 foreigners, have recurred to the opinion, 

 that light consists of vibrations propagat- 

 ed from the luminous body, through a 

 subtle ethereal medium. 



Dr. Franklin, in a letter dated April 

 23, 1752, expresses his dissatisfaction with 

 the doctrine, that light consists of parti- 

 cles of matter continually driven off' from 

 the sun's surface, with so enormous a 

 swiftness. "Must not,'* says he, "the 

 smallest portion conceivable have, with 

 sdch a motion, a force exceeding that of 

 a twenty-four pounder discharged from a 

 cannon ? Must not the sun diminish ex- 

 ceedingly by such a waste of matter ; and 

 the planets, instead of drawing nearer to 

 him, as some have feared, recede to great- 



