3 6 SCIENCE PROGRESS 



this rotatory polarisation by supposing that the incident light is 

 split up by the quartz into two circularly polarised components 

 which travel through it with different velocities ; upon emergence 

 these components recombine, forming again plane polarised 

 light, but owing to the difference of phase introduced by the 

 unequal velocities of propagation, the emergent light is polarised 

 in a plane making an angle with that of the incident light. This 

 angle is directly proportional to the difference of phase caused 

 by the passage through the quartz. If, for any reason, the 

 phase difference changes, an alteration in the amount of rotation 

 will be the result. Suppose, for instance, the velocities of pro- 

 pagation of the two circularly polarised components to be in- 

 creased by the same amount. Their difference will be the same 

 as before; but the phase difference introduced in traversing a 

 constant length of quartz will be reduced by this uniform in- 

 crease of velocity. On the other hand, if both velocities are 

 reduced by equal amounts, the phase difference is increased in 

 the same length of quartz. Such equal changes of velocity may 

 be expected to result from a drift of the ether in the quartz 

 along its axis ; and, on this assumption, there should be a 

 difference of rotatory polarisation according to whether the 

 ether-drift is in the direction of the beam or directly opposed to 

 it. It is true that there is reason to suppose that, in material 

 substances, the velocity of ethereal motion differs from that in 

 vacuo ; but for ordinary materials — as will be shown later — the 

 two are quite of the same order of magnitude. Lord Rayleigh 

 was able, by means of carefully observing the rotation of the 

 plane of polarisation produced by five thick quartz crystals in 

 series between two nicols, to show that its amount was inde- 

 pendent of the supposed direction of ether-drift. 



Before concluding this review of experiments on ether-drift, 

 it will be well to devote a paragraph to what is known with 

 regard to the internal effects of moving matter. If the existence 

 of an ether having the properties ascribed to it be admitted, it 

 cannot be doubted that those parts of it, which actually occupy 

 the same space as material substance, are considerably modified 

 by the presence of the latter. For instance, the ether-waves 

 which constitute light travel more slowly in air than in vacuo, 

 and more slowly still in such media as water and glass. Material 

 substances, although not themselves conveying the luminous 

 vibrations, have the power of reducing the rate at which they 



