PHYSICS OF THE NINETEENTH CENTURY.-LIGHT. 469 



between the first slice and the eye. It is now no longer a matter of 

 indifference as to the position of the axial direction. When the axes 

 of the crystalline plates are both upright the light is seen through both 

 crystals. If, however, the axis of the second piece is horizontal, the 

 light which has passed through the first plate refuses altogether to pass 

 through the second. In intermediate positions, more or less light will 

 pass according as the axial directions of the crystals are more parallel or 

 perpendicular to each other. It will be obvious that the light which has 

 passed through the first tourmaline is no longer in the same condition as 

 the light from the candle, for the rays have now different properties in 

 different directions. It is impossible to conceive any theory of undula- 

 tions consisting of movements in the direction of the ray only which 

 will explain these phenomena ; and the hypothesis of Newton's, which 

 attributed by a strained analogy polar attractions and repulsions to the 

 particles of light, was entirely unsatisfactory. On the other hand, the 

 theory of transverse vibration lends a ready explanation of the pheno- 

 mena exhibited by the tourmaline plates. We must conceive the vibra- 

 tions as taking place transversely to the direction in which the waves 

 are propagated. This is visibly the case with the waves on water, but 

 here the particles vibrate in vertical planes only, whereas in the undu- 

 lations of the luminiferous ether the vibrations occur in planes passing 

 in every direction through the ray. Thus, suppose a ray of light to 

 enter a room and descend vertically : the vibrations are executed not 

 only in planes north and south, or east and west, but in both those 

 directions, and to the same degree in every other direction. Thus 

 some particles will move north and south, others east and west, others 

 north-east and south-west, and so on. Let us consider for the present 

 only two planes of vibrations perpendicular to each other as, for 

 instance, those directed north-south and east-west. The motions of 

 the ethereal particles which take place in other planes can, by the well- 

 known propositions of the resolution of velocity 

 and of forces, be referred to the two planes. ^j- 



Let N s and w E in Fig. 211 represent the direc- 

 tions of these planes," and A B the vibration of a 

 particle in a plane inclined to N s and w E. 



From A draw the perpendiculars A#, A<?, and from w- 



B the perpendiculars B w t B s. Then n s and w e \^^ 

 will represent the mechanical effects of that B " 

 vibration in the planes N s and w E. Now, 

 as the ordinary principles of mechanics are ap- 

 plied to the motions of the ether particles, it FIG. 211. 



follows that, if from some cause the particles 



should at some point in a ray of light be compelled to confine their 

 vibrations to only the two perpendicular planes N s ana w E, the vibra- 

 tions before executed in planes inclined to these would exercise their 

 mechanical effects by the components ns, we, resolved in these two 



