307 



may be acted on in consequence of the conducting power of the 

 medullary substance, more energetically. The muscular sensa- 

 tions being more distinct, and the acts of the will being more en- 

 ergetic, the mind acts on the muscles thus supplied with more 

 power and precision, and recollects and repeats the action, or suc- 

 cession of actions, with more certainty and uniformity than it 

 otherwise could have done. 



2. On the Plane and Angle of Polarization at the Surfaces of 

 Crystals. By Professor Kelland. 



The subject of Crystalline Reflexion has been treated by Mr 

 MacCullagh and M. Neumann ; but both these writers commence 

 their investigations by assuming that certain equations hold true 

 at the common surface of two media. The object of the present 

 Memoir is to obtain such equations from mechanical considera- 

 tions. To attain this, the following hypotheses are made : — 



1. That common light consists of vibrations, whose plane of mo- 

 tion is continually changing without law, or is indeterminate. 



2. That light, polarized in a given plane, consists of vibrations 

 perpendicular to that plane. 3. That the media consist of par- 

 ticles at sensible distances from each other, and acting mutually 

 by a force which varies inversely as the square of the distance. 

 4. That, at the surface, a portion of the motion is destroyed by 

 the sudden change from a motion in one direction to a motion in 

 another. The nature of the medium which constitutes a crystal is 

 defined by the direction which the ray takes within the crystal, as 

 compared with its direction without. By means of these hypo- 

 theses, it is shewn that the following conditions result : — 1. That 

 the resolved parts of the motion in three directions are at the 

 surface the same, whether estimated as belonging to the rays 

 without or within the crystal ; and, 2. That three other equations, 

 derived from the former by a simple law of derivation, coexist 

 with the above, and with them constitute the equations of rela- 

 tion between the incident, reflected, and refracted rays. These 

 equations being solved give the position of the plane of polariza- 

 tion as determined by M. Neumann and Mr MacCullagh. They 

 give also the polarizing angle in a form slightly differing from 

 that obtained by the latter; but differing from it in such a manner 

 as to agree (in the point of difference) with the formula of the 

 former. 



