1 24 CORRELA TION OF PHYSICAL FOR CES. 



At the utmost, our assumption, on the one hand, is, 

 that wherever light, heat, &c., exist, ordinary matter exists, 

 though it may be so attenuated that we cannot recognise it 

 by the test of gravitation ; and that to the expansibility of 

 matter no limit can be assigned. On the other hand, a 

 specific matter without weight must be assumed, of the exist- 

 ence of which there is no evidence but in the phenomena 

 for the explanation of which its existence is supposed. To 

 account for the phenomena the ether is assumed, and to 

 prove the existence of the ether the phenomena are cited. 

 For these reasons, and others above given, I think that the 

 assumption of the universality of ordinary matter is the least 

 gratuitous. 



Qvcev TL TOV Trai'TOQ Kerov ireXei ove 



A question has often occurred to me and possibly to others : 

 Is the continuance of a luminous impulse in the interplanetary 

 spaces perpetual, or does it after a certain distance dissipate 

 itself and become lost as light I do not mean by mere diver- 

 gence directly as the squares of the distances it travels, but 

 does the physical impulse itself lose force as it proceeds ? 

 Upon the view I have advocated, and indeed upon any undu- 

 latory hypothesis, there must be some resistance to its pro- 

 gress ; and unless the matter or ether in the interplanetary 

 spaces be infinitely elastic, and there be no lateral action of a 

 ray of light, there must be some loss. That it is exceedingly 

 minute is proved by the distance light travels. Stars whose 

 parallax is ascertained are at such a distance from the earth 

 that their light, travelling at the rate of 192,500 miles in a 

 second, takes more than ten years to reach the earth ; so that 

 we see them as they existed ten years ago. The distance of 

 most visible stars is probably far greater than this, and yet 

 their brilliance is great, and increases when their rays are 

 collected by the telescope in proportion ceteris paribus to the 

 area of the object-glass or speculum. There is, however, an 

 argument of a somewhat speculative character, by which light 

 would seem to be lost or transformed, into some other force 

 in the interplanetary spaces. 



Every increase of space-penetrating power in the telescope 



