418 Frof. E. Ketteler on the Dispersion of Light 

 understanding by 6 an absolute constant, we have experiment 

 on our side, inasmuch as it is known that in the grand 



total changes but little with the density. 



Therefore only the value of 8 still remains to be cleared up. 

 For this purpose we will imagine the following experiment 

 realized : — 



Upon the plane dividing-surface of an isotropic substance 

 falls, at the angle of incidence 0, the plane of a linearly polarized 

 wave — that is, a pencil of an infinite number of parallel rays. 

 It will enter the interior without refraction ; and its polariza- 

 tion will remain, within as without, the same. Of the pene- 

 trating rays we take one, and imagine the corporeal particles 

 with which it comes into contact in its path characterized by 

 some external token. 



We then compress or dilate the medium in two directions 

 perpendicular to one another (but which, for simplicity's sake, 

 shall both be parallel to the vibration-plane) unequally. The 

 result is twofold. On account of the unequal axial extension, 

 all the rows of molecules which do not fall into the direction 

 of this axis are rotated a certain measurable angle, and among 

 them the line before indicated, whose previous angle % with 

 one of the force-directions changes into % + S = ^. Secondly, 

 the previously singly refracting medium becomes optically 

 uniaxal for the plane considered ; the refracted (extraordinary) 

 wave-plane now corresponding to the incidence-angle cer- 

 tainly remains parallel to the incident wave-plane ; but the 

 ray belonging to it appears, with respect to the incident (% ), 

 likewise rotated through some angle h. Thus, in consequence 

 of the modification taken, with one and the same line of space 

 % the two new directions ^ + S = ^ and y^ + h — ^ would be 

 associated. If at first one of the axes of pressure is made to 

 coincide with the incident ray so as to make % = 0, then $ = &' 

 also becomes =0. If % be then increased, S and 8 will 

 simultaneously increase : for the vicinity of % = 45° they reach 

 their maximum, and sink again to for ^=90°. 



For this peculiar behaviour of the two directions % and %' 

 (one of which is, besides, conditioned by the coexistence of 

 the other) there is, in my opinion, no other satisfactory solu- 

 tion but just the postulate x = X^ ^o = ^- According to this 

 the angle S between wave-normal and ray (or the virtual 

 vibration-directions corresponding to them) would be the same 

 as the angle between the former direction as that of some 

 particles of the unmodified medium and the direction of the 

 same particles after the modification. Or, in other words, 

 The internal ray corresponding to one determinate external 



