632 HENRY A. KOWLAND 



always more or less opaque, the refraction and dispersion of light, dou- 

 ble refraction, and diffraction, all are explained on this theory with an 

 ease and simplicity wanting in all other theories; and, lastly, an elec- 

 tro-magnetic phenomenon has been discovered, which, when applied to 

 this theory of light, explains the rotation of the plane of polarization 

 produced by a magnet. There is no fact in nature seriously in disagree- 

 ment with this theory, and it serves to connect two of our most impor- 

 tant branches of physics, light and electricity. 



But some physicists say that it is not a true theory, because it is not 

 mechanical, the object of these physicists being to reduce every phe^ 

 nomenon of nature to matter and motion. Whether this is necessary or 

 not I leave to the philosophers. But it is to be noted that the old me- 

 chanical theory that light is a vibration in a medium having the prop- 

 erties of an elastic solid is not entirely at variance with the new theory. 

 The medium we call ether. The electro-magnetic theory says that 

 the waves of light are waves of electric displacement, while the old 

 theory says they are waves of ether. Make electricity and the ether 

 equal to each other and the two theories become one. We have arrived 

 at that hazy and unsatisfactory theory of Edlund that ether and elec- 

 tricity are one, except that by this theory electricity is presented to us 

 as an elastic solid! 



But the ground trembles beneath us, and we shall soon be plunged in 

 the mire of vague speculation if we do not draw back. 



Among the other questions which depend for their solution on the 

 presence of a medium may be mentioned the mutual action of two elec- 

 trified bodies moving in space. It has been found that electricity car- 

 ried through space on a charged body has exactly the same magnetic 

 effect on a stationary magnetic needle as if it had been conducted. 



But when electrified bodies move uniformly forward in space, we can 

 conceive of no mutual effect from such motion unless it is relative to a 

 medium, for we cannot even conceive of absolute motion. 



Assuming the medium to exist, we then know that a positively and a 

 negatively charged body flying through space with the velocity of light 

 would have their electric attraction just balanced by their magnetic re- 

 pulsion, and so would exert no force on each other. 



But it is a most wonderful fact that we have never been able to dis- 

 cover anything on the earth by which our motion through a medium 

 can be directly proved. Carried, as we suppose, by the earth with im- 

 mense velocity through regions of space filled with ether, we have never 

 yet been able to prove any direct influence from this ethereal wind. 



