504 M. Verdet on the Optical Properties developed in 

 Conical armature. 



Lastly, the experiments No. III. and No. IV., and the expe- 

 riments No. VI. and No. VII., furnish a confirmation which 

 ought not to be neglected. In the experiments No. III. and 

 No. IV., I measured the rotations produced by two different 

 thicknesses of the same piece of heavy glass ; if these measure- 

 ments corresponded to equal magnetic actions, the rotations 

 ought to be proportional to the thicknesses, in virtue of the 

 identity of the action of all the plates of the substance. In 

 reality the magnetic actions were not the same in the two expe- 

 riments ; but it is clear, that if the experiments had been well 

 made, the ratios of the rotations to the magnetic actions ought 

 to be proportional to the thicknesses. Now these relations are 

 respectively equal to 2-88 and 1-92; by dividing them by the 

 corresponding thicknesses 37-2 and 26, we obtain the quotient 

 0-077 and 0"074, that is to say, numbers whose differences do 

 not exceed the errors of experiment. The experiments No. VI. 

 and No. VII. relative to the bisulphide of carbon, lead to the 

 same conclusion. The ratios of the rotations to the magnetic 

 actions in the two experiments are 2 - 49 and T72; by dividing 

 them by the corresponding thicknesses 44 and 31, the quotients 

 0056 and 0-055* are obtained. 



Part II f. 



In publishing his discovery of the rotation of the plane of 

 polarization produced by the influence of magnetism, Mr. 

 Faraday stated that the phenomenon manifested itself with the 

 greatest intensity when the direction of the ray of light was 

 parallel to the direction of the magnetic forces, and that it dis- 

 appeared when these two directions were perpendicular to one 



* It would not have been possible directly to compare the rotations pro- 

 duced by two different thicknesses under the influence of the same mag- 

 netic action, at least in the case of heavy glass. The solar light in traver- 

 sing the glass heats it perceptibly, and the glass acquires a perceptible 

 birefracting power in every direction perpendicular to that of the ray of 

 light. Any observation with a thickness perpendicular to the first thick- 

 ness studied is therefore impossible, as long as this birefracting power has 

 not disappeared, and this disappearance often requires more than an hour. 



t From the Annates de Chimie et de Physique, vol. xliii. for January 1855. 



