May 1, 1894.] 



KNOWLEDGE. 



Ill 



THE LUMINIFEROUS ETHER. 



By J, J. Stewakt, B.A.Cantab., B.Sc.Lond. 



IN the earlier theories of electricity and magnetism, 

 "action at a distance" was considered a sufiScient 

 explanation ot the mutual influence of electrified 

 bodies or magnets upon each other. Bodies charged 

 with electricity were stated to attract or repel each 

 other with a force varying inversely as the square of the 

 distance, and no account was given or inquiry made as to 

 the mechanism whereby these attractions and repulsions 

 were excited, though actions of material systems on each 

 other at a distance without any intervening medium seems 

 an unthinkable hypothesis. 



To Faraday is due the fruitful theory that these electrical 

 actions are caused by stresses and strains in a medium 

 surrounding and interpenetrating the electrified bodies, 

 and he mapped out the surrounding space by a set of 

 curving lines of force. He showed that the actual 

 phenomena may be explainei by supposing a tension along 



the lines of force, combined with a pressure at right angles 

 to them. This theory has been further developed by 

 Clerk Maxwell, who showed that it gave a consistent 

 explanation of the behaviour of electrified bodies and of 

 magnets. 



But the existence of some medium filling interplanetary 

 space is demanded by the undulatory theory of light, in 

 order to account for the transmission of light-waves from 

 the heavenly bodies, and from all luminous objects to our 

 eyes. Many facts go to prove that the dark radiations, 

 which do not affect our sense of sight but which produce 

 heat in the matter on which they fall, are conveyed by the 

 same medium, and that the heat-producing waves differ 

 only from the luminous vibrations in having a greater 

 wave-length ; the two radiations therefore seem to be 

 essentially the same, but when the waves are shorter and 

 more rapid than the luminous ones, they do not affect our 

 nerves of sight, though their chemical effects may be 

 observed and their existence made manifest by their photo- 

 graphic action. 



The existence of a medium capable of conveying radia- 

 tion by some sort of periodic vibration being abundantly 

 confirmed by the various phenomena of light, the question 

 arises whether it is not the very same medium whose 

 stresses and strains may account for the observed electric 

 effects. It would simphfy matters much if we had not 

 to suppose separate ethers to accoimt for the various 

 phenomena in the different departments of physical 

 research. These various effects are considered by us 

 separately and placed in different categories because they 

 manifest themselves to us in differing ways, but they may 

 all be due to different forms of energy exerting influence 

 through the same medium. To reduce the explanation of 

 physical phenomena to as few forces as possible is simpler 

 and more satisfying to the mind, and the tendency of 

 physical research and modern speculation is to form wider 

 and more comprehensive generalizations, which enable us 

 to include various and apparently disconnected phenomena 

 under the same far-reaching laws. For the departments 

 of electricity and light this has been to a great extent done 

 by the famous electro-magnetic theory of Clerk Maxwell, 

 which considers light as an electro-magnetic phenomenon, 

 and explains the vibrations of light as consisting of some 

 sort of alternate electric polarizations of the particles of 

 the luminiferous ether. 



Let us consider a little what is known of this all- 

 pervading medium, through which are manifested all the 

 remarkable effects of electricity and light and radiation. 



Maxwell's theory has recently received a striking 

 confirmation and illustration by the brilliant experiments 

 of Dr. Hertz, who has investigated the behaviour of long 

 electro-magnetic waves and has not only found that they 

 are capable of reflection, refraction and polarization, 

 but measured their wave-length. He finds that their 

 behaviour is quite analogous to the behaviour of light- 

 waves, and that they differ merely in having a greater 

 length. 



It is possible that the attraction of gravitation may also 

 be due to stresses in the ether, though the cause of gravi- 

 tation no one has yet been able to explain. It is highly 

 probable that if we were acquainted with all the properties 

 of the luminiferous ether, the knowledge would include an 

 explanation of the mechanism of gravitation. 



Newton emphatically rejected the idea of action at a 

 distance. He says in his Letters to Bentley : " You some- 

 times speak of gravity as essential and inherent to matter. 

 Pray do not ascribe that notion to me ; for the cause of 

 gravity is what I do not pretend to know, and therefore 

 would take more time to consider of it. It is inconceivable 



