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SCIENCE. 



[N. S. Vol. I. No. 6. 



which stand for no reality ? In answer to 

 such questions we should have to admit that 

 most of our evidence is what would be called 

 indirect, or circumstantial. Nevertheless, 

 we could maintain that the evidence of 

 things unseen may be very strong, and that 

 it is nowhere stronger than in the domain 

 of the mechanics of the ether. It seems es- 

 sential, therefore, to recall, briefly, the 

 salient features of this evidence. 



In the first place, it is known that light 

 travels through the celestial regions with a 

 definite speed of about 186,000 miles a sec- 

 ond. Induction from a wide varietj' of ob- 

 servations leads also to the conclusion that 

 heat travels with the same speed, and that 

 it and light are, in fact, only different aspects 

 of the same phenomenon. Year in and 

 year out our astronomical tables proceed on 

 the assumption that eight minutes and 

 seventeen seconds after the sun has risen 

 above the plane of our hoi'izon, we may per- 

 ceive his light and feel the glow of his heat. 

 The earth is traveling in its orbit around the 

 sun at the rate of about eighteen miles in a 

 second, a fact which, taken ia connection 

 with the speed of propagation of light, 

 makes the apparent position of a star a lit- 

 tle different fi-om its real position. This is 

 the beatiful phenomenon of aberration dis- 

 covered by the astronomer Bradley more 

 than two generations ago. The impi-essive 

 feature of the phenomenon lies in the fact 

 that it is always the same, due allowance 

 being made for the speed and direction of 

 the earth's motion. Thus we are forced to 

 the conclusion that the velocity of light lq 

 the stellar spaces is the same, regardless of 

 the source and direction of a luminous ray. 

 The step from this conclusion to the con- 

 ception that light is propagated by means of 

 some sort of an elastic medium is easy and 

 natural, and experience with gross matter, 

 like water and air, leads quicklj^ to the sug- 

 gestion that vibration of such a medium 

 must be the mode of propagation . A crowd 



of readily observable facts of reflection, re- 

 fi-action, and diffi-action confirms the sug- 

 gestion and dignifies it with the title hypo- 

 thesis, and finally we are led to accept the 

 undulatory theorj^ of light, and to speak as 

 confidently of the lumiuiferous ether as of 

 any visible matter. Indeed, Lord Kelvin 

 asserted, a few years ago, that we know 

 more of the ether than we do of shoe- 

 maker's wax. Certian it is that the labors 

 of Fresnel, Grreen, Cauchy and their suc- 

 cessors have given us a splendid develop- 

 ment of this mechanical theory of light. 

 But, alas ! they do not enable us to express 

 in common parlance a very definite idea of 

 the medium. No one, it is safe to say, 

 would undertake with any degree of confi- 

 dence to predict how a portion of the ether, 

 a cubic foot say, would look if isolated and 

 rendered Arisible. It naight appear like a 

 very tenuous and tremulous jelly. Its 

 weight would certainly escape detection, for 

 a bulk equal in volume with the earth would 

 M^eigh somewhat less than one ounce. Argu- 

 ing from the phenomena of light alone, it 

 would be found to possess a slight rigidity, 

 but whether it would prove compressible or 

 incompressible we cannot say. 



But the strain on the imagination in trj"^- 

 ing to visualize the ether does not end here. 

 Quite recently it has been rendered almost 

 certain that new and still more complex 

 properties must be attributed to this invisi- 

 ble but omnipresent medium. About thirty 

 years ago, Maxwell, taking up the brilliant 

 experimental researches of Faraday, sought 

 to give mechanical expression to the phe- 

 nomena of electricity and magnetism. The 

 characteristic idea of Faraday and Maxwell 

 concerning these phenomena was that their 

 seat lies not so much in the electrified and 

 magnetized bodies themselves as in some 

 kind of medium surroundmg and permea- 

 ting them. The result of Maxwell's laboi's 

 was the publication, in 1873, of a grand but 

 enigmatic treatise — grand, because of its 



