LIGHT, is that principle or thing by which objects are made perceptible to our sense of seeing ; or the sensation oc casioned in the mind by the view of lu minous objects. The nature of light has been a subject of speculation from the first dawnings of philosophy. Some of the earliest philosophers doubted whe ther objects became visible by means of any thing proceeding from them, or from the eye of the spectator ; but this opinion was qualified by Empedocles and Plato, who maintained, that vision was occasion ed 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 he 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 Newtonian 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 : which rectilinear motion they pre lerve 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 . .
cA a circle, ellipsis, or other curve. For the above properties of the rays of light, see the several words REPILICTIO:q, 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 Roemer, that the eclipses of those satellites happen some times sooner, and sometimes later, than the times given by the tables of them ; and that the observation was before or af 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 Slippose that the light was about 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's difficulty, and left Roemer's conclusion in its full force.