442 THE MODERN THEORY OF LIGHT. 



anytbiug like its i)reseut rate, tbe aspect of physical science becjueatbed 

 to tbe latter half of tbe twentieth century will indeed excite admiration, 

 and when tbe populace are sufficiently educated to appreciate it, will 

 form a worthy theme for poetry, for oratorios, and for great works of 

 art. 



To attempt to give any idea of tbe drift of jirogress in all the direc- 

 tions which I have hastily mentioned, to attempt to explain tbe begin- 

 nings of the theories of elasticity and of matter, would take too long, and 

 might only result in confusion. I will limit myself chiefly to giving 

 some notion of what we have gained in knowledge concerning electric- 

 ity, fether, and light. Even that is far too much ; I find I must con- 

 fine myself principally to light, andouly treat of the others as incidental 

 to that. 



For now well-nigh a century we have bad a wave theory of light ; and 

 a wave theory of light is quite certainly true. It is directly demonstra- 

 ble that light consists of waves of some kind or other, and that these 

 waves travel at a certain well-known velocity, seven times the circum- 

 ference of the earth per second, taking eight minutes on tbe journey 

 from tbe sun to the earth. This propagation in time of an undulatory 

 disturbance necessarily involves a medium. If waves setting out from 

 the sun exist in space eight minutes before striking our eyes, there 

 must necessarily be in space some medium in which they exist and 

 which conveys them. Waves we can not have unless they be waves in 

 something. 



No ordinary medium is competent to transmit waves at anything like 

 tbe speed of ligbt, hence tbe luminiferous medium must be a special 

 kind of substance, and it is called the jetber. Tbe luminiferous tether it 

 used to be called, because the conveyance of light was all it was then 

 known to be capable of; but now that it is known to do a variety of 

 other things also, the qualifying adjective may be dropped. 



Wave motion in petber, ligbt certainly is ; but what does one mean by 

 tbe term wave? The popular notion is, I suppose, of something heav- 

 ing up and down, or perhaps of something breaking on the shore in 

 which it is possible to bathe. But if 3'ou ask a mathematician what he 

 means by a wave, be will probably reply that tbe simplest wave is 



y = a sin {'pt — 71 x) 



and he might possibly refuse to give any other answer. 



And iu refusing to give any other answer than this, or its equivalent 

 in ordinary words, he is entirely justified ; that is what is meant by the 

 term wave, and nothing less general would be all-inclusive. 



Translated into ordinary English the phrase signifies " a disturbance 

 periodic both in space and time .'' Anything thus doubly periodic is a 

 wave ; and all waves, whether in air as sound waves, or in aether as ligbt 

 waves, or on tbe surface of water as ocean waves, arc comprebeuded iu 

 tbe definition. 



