THE ETHER AND MOVING MATTER 107 



on the one condition, viz. that the motion of the ether is differentially 

 irrotational, that if we neglect the square of the aberration and of 

 the time, the change in direction of the ray as it travels along is nil, 

 and therefore the course of a ray is a straight line, notwithstanding 

 the motion of the ether. Following out the analysis on this sup- 

 position, a body, a star for example, will appear displaced toward the 

 direction in which the earth is moving through an angle equal to the 

 ratiq of the velocity of the earth to that of light, when moving normal 

 to the star's direction. This rectilinearity of propagation of a ray, 

 which would likely seem to be interfered with in the motion of the 

 ether, is the tacit assumption made in explaining aberration. If 

 the physical causes, in consequence of which the motion of the ether 

 becomes irrotational, could be adduced, the theory of Stokes would 

 satisfy completely aberration and the negative results of the many 

 and various experimental investigations which have thus far been 

 made and whose validity is unquestioned, whether in refraction, 

 interference, diffraction, rotary polarization, double refraction, 

 induction, electric convection, etc. In an ordinary fluid, tangential 

 forces proportional to the relative .velocities destroy the irrotational 

 condition in a steady state of motion. If we suppose these forces 

 to be diminished indefinitely we obtain now a motion totally differ- 

 ent from that for the steady state when these forces are assumed 

 to be absent initially; and hence such a motion would be unstable. 

 When, however, tangential forces depending on relative displace- 

 ments in the ether are considered, it becomes possible to explain the 

 irrotational condition. Any deviation from this state, for example 

 at a surface of slip, would be dissipated away into space with the 

 velocity of light by means of transverse vibrations. He illustrates 

 such apparent incompatibilities in physical states by successive 

 dilutions of gelatine. Such a medium shows elastic tangential forces 

 for small constraints, and yet apparent fluidity for motions through 

 it, mending itself as soon as dislocated. He regards these qualities 

 as consistent and self-sufficient to explain the phenomena in ques- 

 tion. Against the view of Stokes, Lorentz raises objection to his 

 assumptions concerning the ether motions in the neighborhood of 

 the earth, which he considers inconsistent, a difficulty which he is 

 unable to set aside. Larmor demurs against an appeal to a highly 

 complex medium, such as pitch, for studying the behavior of a 

 simple one like the ether. A time-rate much shorter than the time of 

 relaxation will of course provide approximate rigidity, while a time- 

 rate much longer will provide approximate fluidity, but this requires 

 inevitable dissipation. This objection would be valid for a viscous 

 solid, but such Stokes apparently did not have in mind, since he speci- 

 fically proves such a case unstable. A solid like pitch is a very differ- 

 ent type of solid from that of a vesicular solid like jelly. An ether 



