336 EEPORT— 1893. 



the magnetism is riglit or left handed.' (2187.) ' In all these cases tbe 

 superinduced magnetic rotation was according to the general law, and 

 without reference to the previous power of the body.' 



Further on, after describing the diversity of the effect in different 

 media, and its usually small amount in crystals, he adds : 



' 2182. With some degree of curiosity and hope, I put gold-leaf into 

 the magnetic lines, but could perceive no effect. Considering the extremely 

 small dimensions of the length of the path of the polarised ray in it, any 

 positive result was hardly to be expected.' 



The powerful rotation discovered long after by Kundt, with films of 

 iron, will here be called to mind. 



Repeated trials with various transparent media gave no effect of lines of 

 electro-static force on a ray of polarised light, propagated either along 

 them or at right angles to them. An effect in this case has been detected 

 by Kerr long after, but presented itself as a change in the elasticity, pro- 

 ducing double refraction, and entirely devoid of rotational character. 



' 2224. The magnetic forces do not act on the ray of light directly 

 and without the intervention of matter, but through the mediation of the 

 substance in which they and the ray have a simultaneous existence.' 



Any such changes of internal constitution of media must of necessity 

 (2226) ' belong also to opaque bodies : for as diamagnetics there is no 

 distinction between them and those which are transparent. The degree 

 of transparency can, at the utmost, in this respect only make a distinction 

 between the individuals of a class.' 



After pointing out (2230) that this is ' the first time that the molecular 

 condition of a body, required to produce the circular polarisation of 

 light, has been artificially given,' and is, on that account also, worthy of 

 minute study, Faraday proceeds to draw out in very clear and striking 

 contrast the distinction between the natural undirected rotatory property 

 of liquids like turpentine, and the magnetic property which is related to 

 the direction of the lines of force, as well as the distinction between the 

 latter and the axial but undirected rotatory power of quartz. 



This brief resume of the topics treated in Fai'aday's memoir will be of 

 interest as indicating how thoroughly he probed the problem, and how 

 much his ideas were on the lines of the subsequent development of the 

 subject. 



Mathematical Representations of the Phenomena. 



3. The rotation of the plane of polarisation in quartz and other sub- 

 stances had already been explained by Fresnel as depending on the two 

 principles, (i) that the vibrations which can be propagated without change 

 of form as they proceed are for these substances circular (or it may be 

 elliptical), and (ii) that the velocity of propagation is different according 

 as the vibration runs along in the manner of a right-handed or a left- 

 handed screw-motion. It had also been shown by MacCullagh ' how 

 such properties might be deduced from equations of vibration modified 

 by the insertion of small terms involving (d/dzy, where z is the direction 

 of propagation. 



Soon after Faraday's discovery of magnetic rotation, Airy ^ pointed 

 out the different modifications of the equations of vibration that would 

 similarly account for the existence of the magnetic rotation. 



' J. MacCullagh, Trans. R.I.A., xvii. 1836 ; Collected Works, pp. 63, 186. 

 * G. B. Airy, Phil. Mag., June 1846. 



