348 M . G. Wertheim on the double Refraction 



tity 7T> — which corresponds, in the formulae of M. Neumann, to 



— , — we find it equal to 0*157 by the old formulae, and equal to 

 7 



0*168 by the new ones; this latter differs much less from the 

 value 0*191 which we have obtained by direct experiment. 

 But the difference becomes still less when we take into account 

 that M. Neumann has applied his calculation to the means of 

 measurements taken on two bands which were not of the same 

 description of glass : further, in their composition the glasses of 

 Germany employed by M. Neumann differ sufficiently from those 

 of France to prevent the values of E and of C from remaining 

 the same. A band of Bohemian glass has given me, by means 

 of transversal vibrations, a coefficient of mechanical elasticity of 

 6594, which is notably superior to that of our glasses. 



In general, the theory of flexure itself, and the determination 

 of the changes of volume which occur in all the parts of the 

 body submitted to flexure, seem to me to be based on too many 

 hypotheses with reference to the position of the neutral axis, to 

 serve for the calculation of experiments so delicate as the present ; 

 by following an inverse method, it is my intention to make use 

 of the results which, by purely direct means, 1 have obtained in 

 this memoir, on the examination and elucidation of the theory 

 of flexure. 



Simultaneous effects of a mechanical force and of Magnetism, 



The proximity of a powerful magnet produces in certain iso- 

 tropic bodies a rotation of the plane of polarization. It was 

 interesting to see how this novel action of magnetism, which has 

 been discovered by Mr. Faraday, would be modified when the 

 same body had ceased to be isotropic, and had acquired optic 

 and mechanic axes of different magnitudes. MM. Bertin and 

 Matteucci have made already some researches in this direction. 



It has been seen how the apparatus is disposed for these ex- 

 periments, which have been carried out principally on substances 

 endowed with the rotatoi*y magnetic power, such as the flints. 

 The result has been the same for all substances ; the rotation 

 disappears according as the axes become unequal ; a relatively 

 feeble pressure or traction is sufficient to render the phsenomenon 

 less defined, and aftbrwards it becomes more weakened as we 

 augment the charge; but the moment when it disappears cannot 

 be exactly determined, because the operations lose much of their 

 precision when, in consequence of the newly-produced double 

 refraction, the two images become illuminated. By making use 

 of homogeneous rays, we may in all cases convince ourselves 

 that the rotation has completely disappeared when the difference 



