\S6 An Essay on a Property in LiglU 



such a body of light as does come into the eye, they would oc- 

 cupy the whole of space, and then (to say nothing of the absur- 

 dity of supposing that such a mass of dust could not be felt) they 

 would no longer be transparent; the beam at furthest could not 

 extend a foot beyond the hole through which it entered. Besides, 

 if this dust is light enough to be lifted by tbe air, it may be blown 

 away by a puff of wind, and the experiment is easily tried : if 

 we can dissipate this light with a pair of bellows, it is the dust 

 that produces it; but if we cannot, it must be owing to some other 

 cause. 



By admitting this property to be in light— and I cannot con- 

 ceive how it can be disputed — we have a principle by which we 

 can account for a great many of the phenomena of light, which 

 cannot satisfactorily be explained in any other way. By this 

 jirinciple we are able to account for tbe following phaenomena, 

 without supposing anything but what may be easily conceived, 

 and is extremely probable; which cannot be said in favour of any 

 other hypothesis. 



1. Why light from objects at a distance is ahvays fainter than 

 \\hcn it proceeds from anything that is near. 



2. Why the stars are not visible in the day-time, except from 

 a mine, or the bottom of a well. 



3. The tail of a comet. 



4. The zodiacal light. 



5. The twilight; and, I believe, 



6. The aurora lorealis. 



1. No one will dispute this proposition, that the rays of light 

 grow fainter in proportion to the distance they travel. An apple 

 in the moon (or anything else occupying no more space) viewed 

 through a telescope at a focus where parallel rays meet, ought, 

 by the laws of refraction, to be as much magnified as an apple 

 in the hand ; and if the rays coming from it were equally strong, 

 the colours it gave out would be as fresh, and it would be equally 

 visible. We can only account for the apple not being visible, by 

 supposing that the rays coming from it lose a great part of their 

 substance ; and we cannot account for this loss of substance in a 

 more philosophical way than in the manner I have supposed : if 

 the rays lost no!ie of their substance, distance cannot account 

 for their faintness; if they do, the cause of it is explained. 



It will perhaps be objected to this principle, that if the rays 

 of light lost so much of their substance in travelling the short 

 distance between the moon and our earth, before they arrived at 

 Saturn, or the Georgium Sidus, thev would be entirely extinct, 

 and the sun would be invisible, or dimlv seen to the inhabitants 

 of those planets. In answer to this : If the moon was just twice 

 as far from the sun as the earth is, the angle of the sun's diame- 

 ter 



