246 Wisconsin Academy of Sciences, Arts, and Letters. 



Under the same circumstances, where "heavy glass" would 

 produce a rotation of 6°, Bisulphide of Carbon would produce a 

 rotation of 3° ; flint glass, 2° 8' ; rock salt, 2° 2'; water, 1°. 



The behavior of a large number of substances under the simul- 

 taneous influence of magnetism and circularly polarized light of 

 different colors was examined by Yerdet in 1863. He found the 

 results of his experiment to agree very well with the formula : 



d = mcr — ( I — A — I (1.) 



' X~ \ dX) ^ ' 



where d is the angular rotation of the plane of polarization ; m a 

 constant (the coefficient of magnetic rotation of the medium) ; y 

 the intensity of the magnetic force resolved in the direction of the 

 ray ; c the length of the ray within the medium ; / the wave length 

 in air, of the particular kind of light employed ; i its index of 

 refraction in the medium. 



For Creosote there was considerable deviation from the formula. 

 On account of the mixed nature of Creosote, being an aggregate of 

 Carbolic Acid and several other substances, this might have been 

 expected, even if the above were the true formula representing 

 the relation between the rotation, magnetic force, wave length, 

 and refractive index. 



Verdet has summed up his results as follows : 



1st. " The magnetic rotations of the planes of polarization for 

 light of different colors are approximately as the inverse square 

 of the wave length of the light employed. 



2nd. " The exact law is that the product of the rotation of the 

 square of the wave length, increases from the least refrangible to 

 the most refrangible end of the spectrum." 



3rd. " The substances for which this increase is most sensible 

 are also those which have the greatest dispersive power." 



The formula (1) may be derived from the following more gen- 

 eral formula 



d = -cr^-3. = ^JL^__cA/^l(i-x'Ii\ i-2nCr i^ ....(2.) 



dy vp ^ X^\ dl) ' --^ ^ ^ 



which Prof. Clerk Maxwell has shown to be a consequence of Sir 

 Wm. Thompson's assumption that the only dynamical explana- 



