268 



SCIENCE 



[N. S. Vol. XXX. No. 765 



The interest inspired by equations, too, 

 in some minds is apt to be somewhat Pla- 

 tonic ; and something more grossly mechan- 

 ical— a model, for example, is felt by many 

 to be more suggestive and manageable, and 

 for them a more powerful instrument of 

 research, than a purely analytical theory. 



Is the ether dense or rare? Has it a 

 structure? Is it at rest or in motion? are 

 some of the questions which force them- 

 selves upon us. 



Let us consider some of the facts known 

 about the ether. When light falls on a 

 body and is absorbed by it, the body is 

 pushed forward in the direction in which 

 the light is traveling, and if the body is 

 free to move it is set in motion by the light. 

 Now it is a fundamental principle of dy- 

 namics that when a body is set moving in a 

 certain direction, or, to use the language of 

 dynamics, acquires momentum in that di- 

 rection, some other mass must lose the same 

 amount of momentum ; in other words, the 

 amount of momentum in the universe is 

 constant. Thus when the body is pushed 

 forward by the light some other system 

 must have lost the momentum the body ac- 

 quires, and the only other system available 

 is the wave of light falling on the body; 

 hence we conclude that there must have 

 been momentum in the wave in the direc- 

 tion in which it is traveling. Momentum, 

 however, implies mass in motion. We con- 

 clude, then, that in the ether through which 

 the wave is moving there is mass moving 

 with the velocity of light. The experiments 

 made on the pressure due to light enable 

 us to calculate this mass, and we find that 

 in a cubic kilometer of ether carrying light 

 as intense as sunlight is at the surface of 

 the earth, the mass moving is only about 

 one fifty-millionth of a milligram. We 

 must be careful not to confuse this with the 

 mass of a cubic kilometer of ether; it is 

 only the mass moved when the light passes 

 through it ; the vast majority of the ether is 



left undisturbed by the light. Now, on the 

 electro-magnetic theory of light, a wave of 

 light may be regarded as made up of groups 

 of lines of electric force moving with the 

 velocity of light ; and if we take this point 

 of view we can prove that the mass of ether 

 per cubic centimeter carried along is pro- 

 portional to the energy possessed by these 

 lines of electric force per cubic centimeter, 

 divided by the square of the velocity of 

 light. But though lines of electric force 

 carry some of the ether along with them as 

 they move, the amount so carried, even in 

 the strongest electric fields we can produce, 

 is but a minute fraction of the ether in 

 their neighborhood. 



This is proved by an experiment made 

 by Sir Oliver Lodge in which light was 

 made to travel through an electric field in 

 rapid motion. If the electric field had 

 carried the whole of the ether with it, the 

 velocity of the light would have been in- 

 creased by the velocity of the electric field. 

 As a matter of fact no increase whatever 

 could be detected, though it would have 

 been registered if it had amounted to one- 

 thousandth part of that of the field. 



The ether carried along by a wave of 

 light must be an exceedingly small part of 

 the volume through which the wave is 

 spread. Parts of this volume are in mo- 

 tion, but by far the greater part is at rest ; 

 thus in the wave front there can not be 

 uniformity, at some parts the ether is 

 moving, at others it is at rest — in other 

 words, the wave front must be more analo- 

 gous to bright specks on a dark ground 

 than to a uniformly illuminated surface. 



The place where the density of the ether 

 carried along by an electric field rises to its 

 highest value is close to a corpuscle, for 

 round the corpuscles are by far the strong- 

 est electric fields of which we have any 

 knowledge. We know the mass of the cor- 

 puscle, we know from Kaufmann's experi- 



