LIGHT. 



cd by particles continually flying; off from 

 the surfaces of bodies, which met with 

 others proceeding from the eye ; while 

 the effect was ascribed by Pythagoras 

 solely to the particles proceeding from 

 the external objects, and entering the pu- 

 pil of the eye. But Aristotle -defines 

 tight to be the act of a transparent body, 

 considered as such ; and he observes, that 

 light is not fire, nor yet any matter ra- 

 diating from the luminous body, and 

 transmitted through the transparent one. 



The Cartesians have refined considera- 

 bly on this notion ; and hold that light, as 

 it exists 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 mo- 

 tion by a body ignited, or otherwise pro- 

 perly 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 

 motion, which, by very quick pulses, is 

 constantly compressing the subtle matter 

 between the luminous body and the eye, 

 and excites vibrations of pression : as 

 these vibrations are greater, the body ap- 

 pears 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, hut consists of a 

 great number of very small particles, 

 thrown off from the luminous body by a 

 repulsive power, with an immense velo- 

 city, and in all directions. And these par- 

 ticles, it is also held, are emitted in right 

 lines : whicli rectilinear motion they pre- 

 ^erve till they are turned out of their 

 path by some of the following causes, 

 viz. by the attraction of some other body 

 near which they pass, which is called in- 

 flection, or by passing obliquely through 

 a medium of different density, which is 

 called refraction ; or by being turned 

 aside by the opposition of some interven- 

 ing body, which is called reflection ; or, 

 lastly, by being totally stopped by some 

 substance into which 'they penetrate, and 

 which is called their extinction. A suc- 

 cession 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 in- 

 flection, always preserves a rectilinear 

 course, till it be again changed ; neither 

 is it possible to make it move in the arch 

 of a circle, ellipsis, or other curve. For 

 lire above properties of the rays of light, 



see the several words REFRACTION, RE- 

 FLECTION, &c. 



The velocity of the rays of light is truly 

 astonishing, amounting to nearly two hun- 

 dred thousand miles in a second of time, 

 which is about a million times greater 

 than the velocity of a cannon ball. And 

 this amazing motion of light has been ma- 

 nifested in various ways, and first from 

 the eclipses of Jupiter's satellites. It 

 was first observed by Roemcr, that the 

 eclipses of those satellites happen some- 

 times sooner, and sometimes later, thau 

 the times given by the tables of them ; 

 and that the observation was before or ut- 

 ter the computed time, according as the 

 earth was nearer to, or further from Ju- 

 piter, than the mean distance. Hence 

 Roemer and Cassini both concluded, that. 

 this circumstance depended on the dis- 

 tance of Jupiter from the earth ; and 

 that, to account for it, they must suppose 

 that the light was abotit fourteen minutes 

 in crossing the earth's orbit. This con- 

 clusion, however, was afterwards aban- 

 doned, and attacked by Cassini himself: 

 but Roemer's opinion found an able advo- 

 cate in Dr. Halley, who removed Cassini':; 

 difficulty., and left Roemer's conclusion in 

 its full force. 



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 bo 

 according to the tables ; but when tho 

 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 instaii : 

 taneous, 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 be annihilated, AVC 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 tiii 

 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 di* 

 tances shall be known ; th;se distances 

 are, hovrev-r, ?> imtnen^V tv:i<. that 



