230 FRESNEL. 



In all times and all countries, we find morose disposi 

 tions, who, though ready enough to proclaim the glories 

 of the dead, do not treat their contemporaries with any 

 thing like the same favour. As soon as a discovery is 

 announced, they deny its truth : they contest its novelty, 

 and pretend to detect it in some passage of an ancient 

 writer, obscure and forgotten ; or, lastly, they maintain 

 that it was only the result of chance. 



I do not know whether the men of our age are better 

 than their predecessors : but certainly no doubt has been 

 raised either as to the accuracy, or the novelty, or the 

 importance of the discoveries of which I have just given 

 an account. As to the effect of chance, the blindest envy 



plane p, pass along the axis of rock crystal c, of the thickness T : it 

 emerges polarized in a new plane P , inclined to p, by a certain angle. 

 If the crystal were of a greater thickness T X , the plane wotild be turned 

 still further into the position p//, at T into P 7// , and so on. Thus the 

 successive planes of polarization formed a twisted surface like a cork 

 screw staircase. In some crystals this twisting takes place towards 

 the right, in others towards the left. The change of plane is also dif 

 ferent for each of the different primary coloured rays. Thus exam 

 ined by an analyzer, the transmitted ray always presents a succession 

 of colours. 



Sir J. Herschel showed that the right or left handed character of 

 the polarization agreed with the like inclination of the small facets of 

 the complete crystal round the summit. Biot and Seebeck discovered 

 the same property to exist in certain liquids such as oil of turpentine, 

 and even in some vapours. 



The phenomenon is explained theoretically by supposing two rays, 

 each circularly polarized in opposite directions, traversing the axis 

 together, but with unequal velocities. In this case it is shown me 

 chanically that the resultant of such vibrations will be a plane 

 vibration in a continually changing direction, proportional to the 

 retardation which one of the rays has undergone, behind the other, 

 in traversing successive thicknesses. This was the discovery of 

 Fresnel. For rays deviating a little from the direction of the axis, 

 Mr. Airy showed that a similar theory would apply with ettipticatty 

 polarized light. 



