1892.] 



a Discussion concerning Ether and Matter. 



99 



phase, except that appropriate to altered wave-length ; a change 

 of intensity corresponding to different wave-lengths. 

 Medium alone moving, or source and receiver moving together, 

 gives no change of colour ; no change of direction ; a real lag of 

 phase, but undetectable without control over the medium ; a 

 change of intensity corresponding to different distances, but 

 compensated by change of radiating power. 

 Receiver alone moving gives an apparent change of colour ; an 

 apparent change of direction ; no change of phase, except that 

 appropriate to extra virtual speed of light ; change of intensity 

 corresponding to different virtual velocity of light. 

 The probable absence of a first order effect of any kind, due to 

 ethereal drift or relative motion between earth and ether, makes it 

 necessary to attend to second order effects. 



The principle of least time is applied, after the manner of Lorentz, 

 to define a ray rigorously and to display the effect of existence or 

 non-existence of a velocity potential. Fresnel's law is seen to be 

 equivalent to extending the velocity potential throughout all trans- 

 parent matter. 



It is shown that a ray traversing space or transparent substances 

 will retain its shape, whatever the motion of the medium, so long as 

 that motion is irrotational, and that in that case the apparent direc- 

 tion of objects depends simply on motion of observer ; but, on the 

 other hand, that if the earth drags with it some of the ether in its 

 neighbourhood, stellar rays will be curved, and astronomical aberra- 

 tion will be a function of latitude and time of day. 



The experiment of Boscovich, Airy, and Hoek, as to the effect of 

 filling a telescope-tube with water, does not discriminate between 

 these theories. For if the ether is entirely non- viscous and has a 

 velocity potential, stellar rays continue straight, in spite of change of 

 medium (or at oblique incidence are repeated in the simple manner), 

 and there will be no fresh effect due to change of medium ; while, if, on 

 the contrary, the ether is all carried along near the earth, then it is 

 stationary in a telescope tube, whether that be filled with water or 

 air, and likewise no effect is to be expected. In the case of a viscous 

 ether, all the difficulty of aberration must be attacked in the upper 

 layers above the earth; all the bending is over by the time the 

 surface is reached. It is difficult to see how an ethereal drift will not 

 tend to cause an aberration in the wrong direction. 



Of the experiments hitherto made by Arago, Babinet, Maxwell, 

 Mascart, Hoek, and perhaps others, though all necessary to be tried, 

 not one really discriminates between the rival hypotheses. All are 

 consistent either with absolute quiescence of ether near moving bodies, 

 or with relative quiescence near the earth's surface. They may be 

 ;said, perhaps, to be inconsistent with any intermediate position. 



H 2 



