which hilherto has leen unobserved ly Philosophers. 185 



bv the particles of air. It is evident however that they do not; 

 because, if they reflected all colours alike, the sky would be white, 

 and not blue; and if, as they do, they reflected the blue rays in 

 much greater abundance than all the rest, the looking-glass, as 

 well as the sky, would be blue, and not white ; for it is impossible 

 that the same cause could produce such different phsenomena*. 



I can imagine only one principle that will account for this uni- 

 versal diifusion of litjht, without opposing some other of its phe- 

 nomena, or violating some of the known laws of nature. The 

 particles of light, the moment they are emitted from a luminous 

 body, mav emit other rays of a similar but fainter character; in 

 the same manner, these again may emit others still fainter; and 

 so on, till the stock is exhausted and the light entirely dissi- 

 pated. 



That the rays of light do transmit other rays, in the maimer I 

 have supposed, is proved by a beam of light being visible, in a 

 dark room, when it is crossing and not entering the eve. There 

 is here no surface to reflect the light into the eye, unless the 

 particles of air be considered as such, and then the beam ought 

 to be blue, and not white. 



I know that it has been attempted to account for this beam 

 of light being visible, by supposing that the minute particles of, 

 dust which are light enough to be lifted by the air, and are float- 

 ing in tlie beam, reflect this light into the eye. But, admitting 

 that these particles do reflect light into the eye, they can only 

 reflect light from the spaces which thev themselves occupy; and 

 therefore, unless we can suppose that the whole beam is nothing 

 but a mass of dust, the moment they moved from one place to 

 another, if light still came into the eye from the places they had 

 left, it is clear that there must be light brought into the eye in- 

 dependent of these particles, and that can only be by the agency 

 of the light itself. Glass reflects light only from its surfaces, and 

 that but partiallv, for it suffers a great portion of light to pass 

 through it; when it is pounded into dust it becomes opake, be- 

 cause its surfaces are so infinitely multiplied that not a particle 

 of light can escape and pass through the whole ; and it is whiter 

 than it was before, because more light is consequently reflected 

 from itf. Now, by the same rule, if there were only a few par- 

 ticles of dust floating in the beam, supposing that they did re- 

 flect light, they would appear like stars surrounded by darkness; 

 which is not the case : and if they were thick enongh to reflect 



* If our atmosphere reflects light, we must suppose that the moon's does 

 aUo ; in that case, it would be the moon's atmosphere, apd not herself that 

 \rp hchold ; and what then becomes of the mountains astronomers have dis- 

 covered u()on her glebe ? 



•f Thl? is the reason why snow is whiter than either water or ice. 



such 



