Sir I. Newton long ago observed, that bodies and light act mutually on one an other; bodies on light, in emitting, re. fleeting, refracting, and inflecting it ; and light on bodies, by heating them, and put ting their parts into a vibrating motion,, in which heat principally consists. For all fixed bodies, he observes, when heat ed beyond a certain degree, do emit light, and shine.
This action of bodies on light is found to exert itself at a sensible distance, though it always increases as the distance is diminished, as appears very sensibly in the passage of a ray between the edges of two very thin planes, at differ ent apertures ; which is attended with this peculiar circumstance, that the at traction of one edge is increased as the other is brought nearer it.
The rays of light, in their passage out of glass into a vacuum, are not only in flected towards the glass, but if they fall too Obliquely, they will revert back again to the glass, and be totally reflected. Now the cause of this reflection cannot aye attributed to any resistance of the va cuum, but must be entirely owing to some force or power in the glass, which at. tracts or draws back the rays as they were passing into the vacuum. And this appears further from hence, that if you wet the back surface of the glass with water, oil, honey, or a solution of quick silver, then the rays, which would other wise have been reflected, will pervade and pass through that liquor ; which shows that the rays are not reflected till they come to that back surface of the glass, nor even till they begin to go out of it ; for if at their going out they fall into any of the aforesaid mediums, they will not then be reflected, but will persist in their former course, the attraction of the glass being in this case counter balanced by that of the liquor.
M. Maraldi prosecuted experiments similar to those of Sir 1. Newton, on in. fleeted light. And his observations chiefly respect the inflection of light towards other bodies, by which their shadows are partially illuminated. Acad. Paris, 1723, Mem. p. 159. Sce also Priestley's list. p. 521, &c.
Prom the mutual attraction between the particles of light and other bodies, arise two other grand phenomena, besides the inflection of light, which are called the reflection and refraction of light. It is well known that the determination of bodies in motion, especially elastic ones, is changed by the interposition of other bodies in their way; thus also light, im pinging on the surfaces of bodies, should he turned out of its course, and beaten back or reflected, so as, like other strik ing bodies, to make the angle of its re flection equal to the angle of incidence. This, it is found by experience, light does; and yet the cause of the effect is different from that just now assigned, for the rays of light are not reflected by striking, on the very parts of the reflecting bodies, but by some power equally diffused over the whole surface of the body, by which it acts on the light, either attracting or repelling it, without contact : by which same power, in other circumstances, the rays are refracted ; and by which also the rays are first emitted from the luminous body ; as Newton abundantly proves by a great variety of arguments. See REFLEC