466 Proceedings of Philosophical Societies. [June, 



nished him with the means of foretelling the colour from the 

 thickness. 



In liis second memoir he explains the formulas which his expe- 

 riments had suggested. He shows the kind of polarization which 

 they indicate. These do not take place merely in thin plates, 

 for the author has shown the method of developing long series of 

 cjlours in plates several centimetres thick, by the increase of their 

 axes. He gives a general expression for these phenomena. He 

 shows that in the crystal that produce them, the luminous molecules 

 perform, round thtir centres of gravity, oscillations, of which he 

 determines the extent, the duration, the velocity, as well as the 

 law of the forces that produce them ; and coming from this prin- 

 ciple to the phenomena, he finds by this inveise method of pro- 

 ceeding the very same formulas which he had at first drawn directly 

 from observations. 



He establishes by experiment the way in which the oscillations 

 must continue in passing from one plate to another. Then he de- 

 duces from the theory all the phenomena of colours which plates 

 laid above one another can produce, supposing their axes to cross at 

 any angle whatever, when they are exposed perpendicularly to a 

 polarized ray; and he shows that all these corollaries agree perfectly 

 with observation. 



He deduces equally from his theory the phenomena which take 

 place in case of oblique incidence both by refraction and reflection. 

 For the plates parallel to the axis of crystallization the agreement 

 with experiment is complete. He extends his results to the cases 

 when the axis is inclined to the surfaces of the plates at any angle 

 whatever. 



He observes the peculiar phenomena which plates of rock crystal 

 present when cut perpendicularly to the axis of crystallization, and 

 exposed to a ray polarized under a perpendicular incidence. He 

 shows that the forces which produce these phenomena are inde- 

 pendent of those which produce the oscillatiuns. He proves, by 

 numerous and exact experiments, that the luminous molecules ac- 

 quire in traversing these plates peculiar properties which they re- 

 tain afterwards when they pass into space, and which do not consist 

 merely in a new disposition of their axes, but in a true physical 

 TOodificatioa. The forces which then act upon tliem do not make 

 them oscillate, but turn in a continued molion round their centre of 

 gravity. He explains the laws of this rotation ; he shows how it 

 may be succesively directed from right to left and from left to I'ight. 

 Finally, by inclir.ing the plates, he shows how it is opposed and 

 destroyed by the force which produces the oscillations when this 

 force can develope itself. 



He slio.s that in plates of mica, when they are regularly crys- 

 tallized, the polarizing forces emanate from two lines or axes, one 

 of which is situated in the pFane of the plates and the other is per- 

 pendicular to them'; hence it follows, that the theory of oscillations 

 does not immediately apply to the phenomeaa of mica; as uader 



