CHAP, ix.] ELECTRICITY AND MAGNETISM. 355 



freshly deposited chloride of silver, are placed in a cell 

 with water and connected with a galvanometer, a current 

 is observed to pass when light falls upon one of the two 

 plates, the exposed plate acting as a negative pole. 



39O. Electromagnetic Theory of Light. Clerk 

 Maxwell proposed a theory of the relation between 

 electromagnetic phenomena and the phenomena of light, 

 based upon the assumption that each of these are due to 

 certain modes of motion in the all-pervading " cether " of 

 space, the phenomena of electric currents and magnets 

 being due to streams and whirls, or other bodily move- 

 ments in the substance of the aether, while light is due 

 to vibrations to and fro in it. 



We have seen (Arts. 115, 338, and 387) what evidence there 

 is for thinking that magnetism is a phenomenon of rotation, 

 there being a rotation of something around an axis lying in the 

 direction of the magnetisation. Such a theory would explain 

 the rotation of the plane of polarisation of a ray passing through 

 a magnetic field. For a ray of plane-polarised light may be con- 

 ceived of as consisting of a pair of (oppositely) circularly-polarised 

 waves, in which the right-handed rotation in one ray is periodi- 

 cally counteracted by an equal left-handed rotation in the other 

 ray ; and if such a motion were imparted to a medium in which 

 there were superposed a rotation (such as we conceive to take 

 place in every magnetic field) about the same direction, one of 

 these circularly- polarised rays would be accelerated and the other 

 retarded, so that, when they were again compounded into a 

 single plane-polarised ray, this plane would not coincide with the 

 original plane of polarisation, but would be apparently turned 

 round through an angle proportional to the superposed rotation. 



It was pointed out (Art. 337) that an electric dis- 

 placement produces a magnetic force at right angles to 

 itself; it also produces (by the peculiar action known as 

 induction) an electric force which is propagated at right 

 angles both to the electric displacement and to the mag- 

 netic force. Now it is known that in the propagation of 

 light the actual displacements or vibrations which con- 

 stitute the so-called ray of light are executed in directions 

 at right angles to the direction of propagation. This 



