Existence of the Luminiferous ^Ether. 483 



around two lines in the crystal, while for violet light the ar- 

 rangement is only around one such hue. (It does not make any 

 difference, so far as this is concerned, whether we consider the 

 aether or the molecules concerned in the transmission of the 

 light.) Xow we are thus brought face to face with one of 

 two things, viz. either the different vibrations of the two kinds 

 of light produce differences in the molecular arrangement of 

 the crystal, or the molecular arrangement itself is different for 

 red to what it is for violet light. That a change takes place 

 in the arrangement of the molecules when the different kinds 

 of light fall on the crystal is most unlikely; and thus we are 

 left with the second alternative. The intermolecular aether, 

 then, must be so constituted in these crystals as to be capable 

 of vibration for long waves (red light) in one way, and for 

 short waves (violet light) in another. How is this constraint 

 brought about? TVe are told by the presence of the mole- 

 cules of mundane substance whose molecules are scattered 

 about in this aether just as the stars are in space. It is con- 

 trary to our notions of nature to imagine that the mere pre- 

 sence of the molecules is going to affect the elasticity of the 

 substance in this complicated manner. If, however, we ima- 

 gine the molecules of the crystal concerned in the propagation 

 of light through it, we must make a slight additional assump- 

 tion, viz. that different groups of molecules are concerned in 

 the transmission of different kinds of light. Then we have 

 that the groups concerned in the transmission of the red waves 

 are differently arranged from those which transmit the violet. 

 Thus in the case of the crystal cited we have that the groups 

 of molecules transmitting the red are symmetrically arranged 

 around two axes, while those transmitting the violet are thus 

 arranged around one axis only. 



Thirdly. We take the experiments of Herschel by which he 

 proved that the axes are inclined to each other at different 

 angles for light of different refrangibilities. Thus in Rochelle 

 salt the axes for violet light are inclined to each other at 56°, 

 and for red light at 76°. In explanation of this, Herschel says : — 

 " The directions within the crystal of the two axes of double 

 refraction, or the ' optic axes,' stand in no abstract geometrical 

 relation to those of the angles and edges of its c primitive 

 form,' or to its axes of symmetry. They are resultant lines 

 determined by the law of elasticity of the luminiferous aether 

 within its substance as related to its crystalline form, and to the 

 wave-length of the particular coloured ray transmitted "*. In 

 reference to this, it is only necessary to remark that here we 

 have to make an additional assumption with regard to the 

 * Herschel, ' Familiar Lectures,' p. 398. 



