THE LUMINIFEROUS ETHER. 93 



To give a conception of the immense distance of the fixed 

 stars, I will assume as known that light travels at the rate of 

 about 180,000 miles in one second, a rate which would carry 

 it nearly eight times round and round the earth in that time ; 

 and yet if we take the star which, so far as we know, is our 

 nearest neighboiu', it would take three or four years for light 

 from that star to reach the earth. Now as we see the tixed 

 stars there must be some link of connection between us and 

 them in order that we should be able to perceive them. 

 Probably all of you know that two theories have been put 

 forward as to the nature of light, as to the nature accordingly 

 of that connection of which I have spoken. According to 

 one idea, light is a substance darted forth from the luminous 

 body with an amazing velocity ; according to the other, it 

 consists in a change of state taking place, propagated through 

 a medium, as it is called, intervening between the body from 

 which the light proceeds and the eye of the observer. For 

 a considerable time the first of these theories was that chiefly 

 adopted by scientific men. It was that, as you know, which 

 Newton hiuiself adopted ; and probably the prestige of his 

 name had much to do with the favourable recejDtion which 

 for a long time it received. But more recent researches have 

 so completely established the truth of the other view, and 

 refuted the old doctrine of emissions, that it is now univer- 

 sally held by scientific men that light consists in an undula- 

 tory movement propagated in a medium existing in all the 

 space through which light is capable of passing. 



This necessity for filling all space, or at least, such an 

 inconceivably great extent of space, with a medium, the 

 office of which, so far as was known in the first instance, 

 was simply that of propagating light, was an obstacle for a 

 time to the reception by the minds of some of the theory of 

 undulations. Men had been in the habit of regarding the 

 inter-planetcuy and inter-stellar space as a vacuum, and it 

 seemed too great an assumption to fill all this supposed 

 vacuous space with some kind of medium for the sole pur- 

 pose of transmitting light. Notwithstanding, even long 

 ago strong opinions were entertained to the effect that 

 there must be something intervening betAveen the different 

 heavenly bodies. In a letter to Bentley, Newton expresses 

 himself in very strong language to this effect : " That 

 gravity should be innate, inherent and essential to matter, 

 so that one body may act upon another at a distance 

 through a vacuum, without the mediation of anything else. 



