which hitherto has leen unobserved ly Philosophers. 18/ 



ter to an inhabitant of the tnoon would only measure half as much 

 as it does to us ; and consequently, the same number of rays from 

 the sun would only occupy half the space that they do here. What 

 each individual ray therefore loses in substance may be made up 

 by additional numbers: the apple in the moon, in consequence, 

 may give out as vivid a colour as the apple in the hand ; but the 

 light from one has only two or three feet to travel, while the 

 other is emitting rays for the space of two hundred and forty 

 thousand miles before any part of it can enter the eye. If we 

 suppose the fixed stars to be suns of equal magnitude with our 

 own, everv star must send down so much more light upon tlie 

 earth than the sun does, in proportion as its angle (which cannot 

 be measured) is smaller than the sun's angle : if the stars there- 

 fore did not lose a very great proportion of their light in conse- 

 quence of the great distance from us, the light of the sun would 

 be lost in the greater splendour of the stars' light, and the light 

 of a single star entering the eye would be intolerable. 



2. If the sun's primary rays did not emit secondary rays in the 

 manner I have described, whenever we turned our eyes towards 

 those parts of the heavens where he was not, the stars would be 

 visible, because there could be no light enter the eye but what 

 came from them. The reason then why we do not see the stars 

 in the day-time is, because these secondary rays going in all direc- 

 tions, strike in great numbers upon the pupil of the eye, and are 

 refracted down to the retina, which they cover with a complete 

 coat of light; and as the light of the stars, travelling so great a 

 distance, is fainter than this light, it cannot be distinguished from 

 the mass ; and the stars, in consequence, are invisible. When a 

 man, however, happens to be at the bottom of a deep well, or 

 coal-mine, a great part of these secondary rays are intercepted 

 by the sides of the well ; the coat of light on the retina is then 

 fainter than the light of the stars j the sky, in con^^equence, looks 

 dark, and the stars are visible. 



If it be objected that these secondary rays, coming into the 

 eye from every direction, would create a great deal of confusion, 

 and prevent us from perceiving any object distinctly; I answer, 

 that we can only account for the same object being visible to dif- 

 ferent persons at the same time, by supposing that the rays of 

 light or colour, coming from all objects, are crossing each other 

 in every «lirection; and therefore this can make no argument 

 against my position, even if it could not be answered. But, in 

 fact, no confusion can arise from tliis diffusion of light, because 

 the primary rays reflected from any object are stronger than the 

 secondary, and the secondary than those in the third degree: 

 thus, for instance, if we look towards a red spot on a bhic ground, 



both 



