1S4 An Essay on a Properly in Light 



at least, only so much of them would he visible as reflected the 

 rays into the eye by their proper angles ; we should see no more 

 of the glass than that part of it upon which tlie in)age of the 

 candle was expressed, the moon would be a speck *, the planets 

 invisible; and, whenever we turned our backs upon the sun, as 

 all his light would be reflected from and not into the eye, we 

 should be enveloped in darkness even at noon day. 



"There is no glass or speculum," says Sir Isaac Newton in 

 his Treatise on Optics, " iiow well soever polished, but, besides 

 the light which it refracts or reflects regularly, scatters every 

 way irregularly a faint light, by means of which the polished 

 surface, when illuminated in a dark room by a beam of the sun's 

 light, may be easily seen in ail positions of the eye." 



Now, without stopping to examine whether the irregulari- 

 ties on the moon's and the earth's surfaces would not rather 

 exhibit a patchwork of light and darkness than a regular diffu- 

 sion of light, by means ol which all objects would be equally vi- 

 sible, it is easy to show that there are many situations in which 

 light is present, when its appearance cannot be explained by any 

 such hypothesis. The only opening, for instance, in a chamber, 

 by whicii light can l)e admitted, is a window facing the east, and 

 the sun is setting in the west. This window, moreover, overlooks 

 the brow of a steep hill which casts a shadow over all the objects 

 within the visible horizon. Now it is impossible that light, un- 

 der these circumstances, can be reflected into the room, because 

 all the sun's light that would strike upon these ol)jects, and might 

 be reflected into the room, is intercepted by the hill; and there- 

 fore it is to some other principle that we must look for an ex- 

 planation of this phaenomenon. 



It will probably occur to the reader, that this principle may be 

 found in the reflection of the particles of which the atmosphere 

 is composed ; and the idea is not unreasonable, because we can 

 only account for the blue colour of the heavens by supposing that 

 the rays of blue colour are reflected, in this manner, into the eye 



• If two lines are subtended from the moon's centre in an angle of half a 

 degree, they would just touch the surface of the sun, for that is about the 

 angle his diameter measures to an inhabitant of the moon. Now, by the 

 above principle of re<lection, all the direct rays of light from the sun that 

 passed outside of those lines would be reflected outside of the sun, so that 

 only the tlirce hundred and sixtieth part of the moon's diameter could throw 

 back any light into the sun ; and these rays, instead of meeting in a focus, 

 •would diverge and spread over the whole of the sim's hemisphere. It follows 

 therefore that not more th.^n the five or six liundredth part of the moon's 

 diameter could reflect the sun's light into a focus, at the distance this earth 

 is from the moon ; and consequently, the moon's diameter, instead of mea- 

 suring an angle of fifteen or sixteen minutes, would hardly measure half 

 a dozen seconds. 



by 



