LIGHT. 



ed beyond a certain degree, do emit 

 light, and shine. 



This action of bodies on light is {bund 

 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 

 he 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 I. Newton, on in- 

 flected light. And his observations chieily 

 respect the inflection of light towards 

 other bodies, by which their shadows are 

 partially illuminated. Acad. Paris, 1723, 

 Mem. p. 159. See also Priestley's Hist. 

 p. 521, &c. 



From 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 

 be '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 or, 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- 

 TION and UEFUACTTON. 



That great author put it past doubt, 

 that all those rays which are reflected 

 do not really touch the body, though they 

 approach it infinitely near ; and that those 

 which strike on the parts of solid bodies 

 adhere to them, and are, as it were, ex- 

 tinguished and lost. Since the reflection 

 of the rays is ascribed to the action of 

 the whole surface of the body without 

 contact, if it be asked how it happens 

 that all the rays are not reflected from 

 every surface, but that, while some are. 

 reflected, others pass through and are 

 refracted ? the answer given by Newton 

 is as follows : Every ray of light, in its 

 passage through any refracting surface, 

 is put into a certain transient constitution 

 or state, which in the progress of the ray 

 returns at equal intervals, and disposes 

 the ray at every return to be easily trans- 

 mitted through the next refracting sur- 

 face, and between the returns to be easily 

 reflected by it : which alteration of reflec- 

 tion and transmission, it appears, is pro- 

 pagated from every surface, and to all 

 distances. What kind of action or dis- 

 position this is, and whether it consist in a 

 circulating or vibrating motion of the ray, 

 or the medium, or something else, he 

 does not inquire ; but allows those who 

 are fond of hypothesis to suppose that the 

 rays of light, by impinging on any re- 

 flecting or refracting surface, excite vi- 

 brations in the reflecting or refracting 

 medium, and by that means agitate the 

 solid parts of the body. These vibrations, 

 thus produced in the medium, move fast- 

 er than the rays, so as to overtake them ; 

 and when any ray is in that part of the 

 vibration which conspires with its motion, 

 its velocity is increased, and so it easily 

 breaks through a refracting surface ; but 

 when it is in a contrary part of the vibra- 

 tion, which impedes its motion, it is easily 

 reflected ; and thus every ray is succes- 

 sively disposed to be easily reflected or 

 transmitted by every vibration which 

 meets it. These returns in the disposi- 

 tion of any ray to be reflected, he calls 



