Intelligence and Miscellaneous Articles. 319 



such an effect entirely in a current produced by forces of the second 

 or higher orders." It is clear, however, that the Astronomer Royal 

 is right and Mr. Heath wrong. Mr. Heath's mistake is on the same 

 page (185), where he determines the moment tending to destroy the 

 earth's rotation. The moment, according to Mr. Heath, is 



+ "^<*+4+!+i>. 



where 



y= c cos 2oj. 



Mr. Heath has not here seen that in his approximate solution to 

 the first order, y 2 must be neglected, because it is of the same order 

 as the forces of the second order, which have been neglected through- 

 out the solution. If we neglect \f, the moment reduces to zero, as 

 Mr. Airy found. 



March 9, 1867. 



ON THE DIRECTION OF VIBRATIONS IN POLARIZED LIGHT. 

 BY M. MASCART. 



Mr. Stokes* was the first who proposed to use the phenomenon 

 of diffraction in gratings for deciding the important question of the 

 direction of vibrations in polarized light. He observed that if the 

 plane of polarization of the incident light is oblique to the lines of 

 the grating, the diffracted light is polarized in another plane. As- 

 suming the vibrations to be parallel or perpendicular to the plane of 

 polarization, he calculated what ought to be the displacement of the 

 plane of polarization of the diffracted light, and he has thought him- 

 self entitled to conclude that Fresnel's opinion was correct, namely, 

 that the vibrations are perpendicular to the plane of polarization. 

 Holtzmannf replaced the gratings made on glass with a diamond by 

 those made with lampblack ; and the result of his experiments is in 

 disaccordance with the hypothesis of Fresnel. Eisenlohrj:, finally, 

 has considered the question from another point of view, by taking 

 into account the influence of the longitudinal vibrations, w r hich had 

 been hitherto neglected. 



Having at my disposal a grating of unusual perfection, I endea- 

 voured to repeat these experiments by comparing the intensities of 

 the light diffracted at the same distance by two incident pencils, one 

 of which was polarized parallel, and the other at right angles to the 

 .lines of the grating. These two pencils are obtained polarized at 

 right angles by placing before the slit of a collimator two pieces of 

 the same tourmaline the axes of which are crossed, or, better, a crys- 

 tal of Iceland spar whose ordinary and extraordinary pencils are sepa- 

 rated on emerging from the crystal. The grating is placed at right 



* Transactions of the Cambridge Philosophical Society, vol. ix. 

 t PoggendorfFs Annalen, vol. xcix. 

 % Phil. Mag. S. 4. vol. xviii. p. 186. 



