Theory of the Magneto-Optic Phenomena of Iron, $c. 487 



ratio = 0"32, and instead of obtaining equality when the photo- 



1+flE 



meter is adjusted for this value the difference is most marked. 



The effect has also been shown by obtaining two photographs on 

 the one plate; one photograph being the result of an exposure to 

 the light from two fluorescing cubes one behind the other, and 

 the second photograph the result of superposing the effect of the 

 light from A alone, when fluorescing, npon that from B after having 

 passed through A, when the latter was not fluorescing. The exposure 

 in each of the three cases being the same, a very distinct difference 

 is shown in the result ; the superposed photographs being always the 

 darker in the negative, notwithstanding the fact that the resultant 

 effect of superposing two photographs due to light of the same 

 intensity, or nearly so, has been found not to be equal to but less 

 than that due to light of double the intensity acting for half the 

 time. If the resultant effect were equal to the sum of the separate 

 ones, the effect caused by the change of absorption would have been 

 still more marked. 



In the determinations of a and ft a null method has been em- 

 ployed by which any appreciable want of uniformity in the illumi- 

 nation can be detected. 



The source of illumination has been almost invariably the spark 

 discharge of a Ley den jar between cadmium electrodes, being one of 

 the richest sources of the fluorescence-exciting rays, and the photo- 

 meter one specially constructed for the purpose. 



" On the Theory of the Magneto-Optic Phenomena of Iron, 

 Nickel, and Cobalt." By J. G. Leathem, B.A., Fellow of 

 St. John's College, Cambridge. Communicated by Sir 

 Robert S. Ball, F.R.S. Received May 11,— Read June 

 17, 1897. 



(Abstract.) 



In Mr. Larmor's Brit. Assoc. Report (1893) on the Action of 

 Magnetism on Light, it is pointed out (§ 20) that there are two 

 ways in which the magnetic field may affect the phenomena of light 

 propagation, and two corresponding types of magneto-optic theory. 

 It is the object of the present paper to take the fundamental 

 equations of the second type of theory in a general form on the lines 

 of Mr. Larmor's recent papers on Electrodynamics, and to develop 

 them so as to obtain the solutions of the problems of magnetic reflec- 

 tion and of transmission through magnetised films ; the formulas so 

 obtained are compared with the available experimental results, and 

 the agreement of the theory with experiment thus put to the test. 



VOL. lit. 2 M 



