114 THE LUMINIFEROUS ^.THER. 



which 1 have spoken. According to one idea, light is a vsnbstnnce 

 darted forth from the luminous body with an amazing velocity; accord- 

 ing to the other, it consists m a change of state taking place, propa- 

 gated through a medium, as it is called, intervening l)etween the body 

 from which the light proceeds and the eye of the observer. For a con- 

 siderable time the first of these theories was that chiefly adopted by 

 scientific men. It was that, as you know, which Newton himself adopted ; 

 and probably the prestige of his name had nmch to do with the favor- 

 able reception 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 universally 

 hehl by scientitic men that light consists in an undulatory 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-planetary and inter-stellar space as a vacuum, and it seemed too 

 great an assuini)tion to fill all this supposed vacuous space with some 

 kind of medium for the sole purpose of transmitting light. Notwith- 

 standing, even long ago strong oi)inions were entertained to the eftect 

 that there must be something intervening between the difterent heav- 

 enly bodies. In a letter to Beiitley, Newton expresses himself in very 

 strong language to this effect: "That gravity should be innate, inher- 

 ent and essential to matter, so that one body may act upon another at 

 a distance through a vacuum, without the mediation of anything else, 

 by and thr<mgh 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 philoso])hical matters a comiietent faculty of thinking, 

 can ever fall into it. Gravity must be caused by an agent acting con- 

 stantly according to certain fixed laws; but whether this agent be 

 materiai or immaterial, I have left to the consideration of my readers." 



What the^-uature 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 gravi- 

 tation may be — we do not know; for anything we know to the ccmtrary, 

 it may be connected with this intermediate medium or luminiferous 

 a'ther. There are other offices, we believe, which this luminiferous 

 a'ther fulfills, to which I shall have occasion to allude presently. 



In connection with the necessity for filling such vast regions of space 

 with this medium, a curious cpiestion naturally i)resents itself. We 

 can not conceive of space as other than infinite, but we habitually think 

 of matter as occni^ying here or there limited portions of space, as for 

 examx)le the different heavenly bodies. The intervening space we 



