94 SIR G. GABRIEL STOKES^ BART., P.R.S., ON 



by and through which their action and force may be 

 conveyed from one to another, is to me so great an absurdity, 

 that I believe that no man who has in philosophical matters 

 a competent faculty of thinking, can ever fall into it. 

 Gravity must be caused by an agent acting constantly 

 according to certain fixed laws ; but whether this agent be 

 material or immaterial, I have left to the consideration of my 

 readers." 



What the nature of the connection between the earth 

 and the sun. for example, may be whereby the sun is 

 able to attract the earth and thereby keep it in its orbit, in 

 other words, what the cause of gravitation may be, we 

 do not know ; for anything we know to the contrary, it may 

 be connected with this intermediate medium or luminiferous 

 ethc]-. There are other offices, we believe, which this lumini- 

 ferous ether fulfils, to which I shall have occasion to allude 

 presently. 



In connection with the necessity lor filling such vast 

 regions of space with this medium, a curious question 

 naturally presents itself. We cannot conceive of space as 

 other than infinite, but we habitually think of matter as 

 occupying here or there limited portions of space, as for 

 example the different heavenly bodies. The intervening 

 space we commonly think of as a vacuum, and it is only the 

 phenomena of light that ledus in the first instance to think of it 

 as filled with some kind of material. The question naturally 

 presents itself to the mind, is this ether absolutely infinite 

 like space ? This is a question to which Science can give 

 no answer Though we cannot help thinking of 8j)ace as 

 infinite, yet when we turn our thoughts to some material 

 existing in space perhaps we more readily think of it as 

 finite than infinite. But if the ether, however vast the 

 portion of space over which it extends, be really limited, 

 we can hardly fail to speculate what there may be outside 

 its limits. Space there might be Avholly vacuous, or possibly'- 

 outside altogether this vast system of stars and ether there 

 may be another system subject to the same laws, or subject 

 to different laws, as the case may be, equally vast in extent ; 

 and if there be, then so far as we can gather from such 

 phenomena as are open to our investigation, there can be no 

 communication between that vast portion of space in part of 

 which we live and an ideal system altogether outside the 

 ether of which we have been speaking. 



But the properties of tlie etlier are no less remarkable 



