406 Mr. W. J. M. Rankine's General View of an 



movement, is the direction of vibration) is perpendicular to the 

 plane of polarization. 



Hence it follows, that in a crystalline medium the velocity of 

 light depends simply on the direction of the transversal charac- 

 teristic of the movement propagated, and not on the direction 

 of propagation. 



This conclusion is opposed to the laws of the propagation of 

 transverse vibrations through a crystalline elastic solid, or through 

 any medium in which the velocity of propagation depends on 

 elasticity varying in different directions. Therefore the velocity 

 of light depends on something not analogous to the variations 

 of elasticity in such a medium. 



To solve this difficulty, the author of this paper some time 

 since suggested the hypothesis that the luminiferous medium 

 consists of particles forming the nuclei of atmospheres of ordi- 

 nary matter ; that it transmits transverse vibrations by means 

 of an elasticity which is the same in all substances and in all 

 directions ; and that the variations in the velocity of the trans- 

 mission of vibrations arise from variations in the atmospheric 

 load which the luminiferous particles carry along with them in 

 their vibrations, and which is a function of the nature of the 

 substance, and, in a crystalline body, of the direction of vibration. 



But although this hypothesis removes the inconsistency just 

 pointed out as existing within the theory itself, it leaves undi- 

 minished the difficulty of conceiving a medium pervading all 

 space, and possessed of an elasticity of figure, at once so strong 

 as to transmit the powerful energy of light with its enormous 

 velocity, and so feeble as to exercise no direct appreciable effect 

 on the motions of visible bodies. 



2. Statement of the Proposed Hypothesis of Oscillations. 



The hypothesis now to be proposed as a groundwork for the 

 undulatory theory of light, consists mainly in conceiving that the 

 luminiferous medium is constituted of detached atoms or nuclei 

 distributed throughout all space, and endowed with a peculiar 

 species of polarity, in virtue of which three orthogonal axes in 

 each atom tend to place themselves parallel respectively to the 

 corresponding axes in every other atom ; and that plane-polarized 

 light consists in a small oscillatory movement of each atom round 

 an axis transverse to the direction of propagation. 



Such a movement would be transmitted through such a medium 

 with a velocity proportional, — directly, to the square root of the 

 total rotative force exercised by the luminiferous atoms in a 

 given small space, upon those in a given adjacent small space 

 lying in the direction of propagation, in consequence of a given 

 amount of relative angular displacement round the axis of oscil- 



