194 An Essay on a Properly in Light 



more nor less than the sun's secondary rays sending light down 

 into the eye. I have seen it frequently in the East Indies ; and 

 the appearance it puts on is exactly what I should have antici- 

 pated, had I conceived my principle before 1 saw it. It resem- 

 bles very much the stars in the Milky Way, and in the centre 

 near the horizon is very nearly as bright; it is broad at the base, 

 pointed like a leaf, and fades away imperceptibly at the edges. 

 This light always marks the track of the sun, and therefore is 

 evidently derived from him; and that the sun may send his light 

 down in this way is proved by the moon, a fainter light, doing 

 the same thing. The zodiacal light, the tail of a comet, and 

 the stars in the Milky Way are very nearly eclipsed by the 

 greater splendour of the moon's light, though to look at either of 

 these, we must turn our back towards her; and as her direct 

 light in this situation cannot enter the eye, it is impossi- 

 ble to account for this but by supposing that her secondary rays 

 cover the retina with as bright a light as these phaenomena would 

 otherwise exhibit. This ))haenomenon cannot in any way be ac- 

 counted for by supposing that it has anything to do with the sun's 

 atmosphere, for, if the sun's atmosphere did extend so far, it 

 would no longer be the sun's atmosphere : the only reason why 

 cur own atmosphere, water, stones, and other loose bodies do 

 not fly off into the sun, is becaose the earth's attraction, which 

 is close, is stronger than the sun's, which is at a distance ; and 

 therefore, if the sun's atmosphere did come so near to the earth, 

 the earth's attraction would seize hold of a great part of it, and 

 sad confusion would ensue. 



5. The phcenomenon of the twilight bears so close a resem- 

 blance to the zodiacal light, that if my principle be admitted in 

 the one case it will hardly be disputed in the other. The twi- 

 light, like the zodiacal light, is evidently derived from the sun ; 

 for it is always strongest in that part of the horizon which is op- 

 posite to him ; and the only difference between them is, that owing 

 to the greater density of the atmosphere in our climate, the sun's 

 ravs are refracted and pressed closer together, and consequently 

 send down a greater body of light ; and this is the reason why 

 the figure of the twilight is not so perfect as that of the zodiacal 

 light. Although the refraction of the atmosphere will account 

 for the greater light of the twilight and the flatness of ils figure, 

 we cannot upon that principle account for its light being brought 

 into the eye; because, if the primary rays of 'he sun were still 

 refracted into the eye, all the coi.seciuence would be, thai the sun 

 would appear to rise higher and be longer in netting ; for upon 

 no principle of refraction can we account for the sun'b rays di- 

 verging upward being brought down by refraction into the eye, 

 without supposing that those that were not directed so high 



would 



