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 Pp, pass along the axis of rock crystal c, of the thickness T: it 
emerges polarized in a new plane P’/, inclined to Pp, by a certain angle. 
If the crystal were of a greater thickness 1’, the plane would be turned 
still further into the position P//, at 7’ into p//”, and soon. 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 
Fresne]. For rays deviating a little from the direction of the axis, 
Mr. Airy showed that a similar theory would apply with elliptically 
polarized light. 
ee ee 
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