138 Radiant Forces. 



If we receive a quantity which we may term x, a planet twice 

 as far off would receive only one quarter of x quantity ; and one 

 three times as far off one ninth, and so forth, till we come to 

 such remote planets as Uranus and Neptune, when the fraction 

 becomes minute. 



If we adopt the explanation of diverging rays, the sun may 

 be regarded as an infinite number of incandescent points, from 

 each of which proceeds one ray, which we call a direct ray, as it 

 proceeds straight forward towards our earth. Of such rays, only 

 those could reach us which emanated from a surface not larger 

 than our own, or so little larger, that the effect of the refractive 

 action of the ether of space, if such there be, and of the 

 refractive action of our atmosphere, could still bend them so as 

 to reach us. Each particle of the solar photosphere would be 

 supposed to emit one of these direct rays, and an infinite 

 number of rays slanting more or less away from it on each 

 side. Now, however slight may be the divergence of any two 

 rays, there must be a distance beyond which they could not 

 both touch any object of given dimensions. Let us suppose, 

 for example, the case of two rays which diverge at the rate of 

 an inch in a million of miles. They could not both fall upon 

 an object rather more than a million of miles away from their 

 starting-point, and not more than an inch in diameter. At 

 exactly one million of miles' distance they would both meet the 

 edges of such an object, but beyond that distance they would 

 pass by it, and at two millions of miles they could not both at 

 once be nearer than within half an inch of it, though one might 

 touch it, and the other be still further off, if it were appro- 

 priately placed for such a result. 



The proportion of diverging rays which planets could obtain 

 from the sun would follow the rule of inverse proportion that 

 has been explained, but not so the rays that come from him 

 in parallel lines. If they were not obstructed by any medium 

 diffused through space, they would go on for ever without 

 diminution ; and if there were two bodies smaller than the 

 sun, and at gigantic distances from it, the nearest being so far 

 off* that it could not receive any diverging- rays emitted by the 

 sun, and the remotest three times, or ten times, that distance, 

 they would both be equally well illuminated, and would receive 

 equal quantities of light on equal areas of their surface. 



As light may be considered, with approximate certainty, 'to 

 consist of vibrations of a highly attenuated fluid, which has not 

 yet been discovered to possess any weight or sensitiveness to the 

 action of the attraction of gravitation, we must suppose such a 

 fluid diffused through space, and capable of transmitting the 

 light waves. This fluid has been thought to oppose a certain 

 obstacle to the passage of light waves, so that it could not pass 



