8 



Proceedings of the Itoi/al Irish Academy. 



period are not now q,,, but slight distortions of them, which, in 

 the case of q^^ qy, since the rotation is about z, are equivalent to 

 introducing small components at right angles, causing a rotation of 

 their magnetic field with the planes of reference, that is, q^c and q,, 

 may be conceived to rotate with these planes. The circuit g', also 

 rotates, but the distortion due to this cause does not change the direc- 

 tion of the magnetic force, but only its magnitude. The orbit q^ is 



now described with angular velocity ^ - ~, and the magnetic 

 field due to this orbit can be replaced by that due to the circuit 



6 6 



q,, with current strength — and an additional current - where 



denoted by q.\ 



To show that it is only the class of components q.! which need 

 be considered in general, it is necessary to examine the state of 

 things in the absence of an external magnetic field. Assuming that 

 the ions are ever in a state of motion, then their magnetic field can be 

 grouped into the three components which, summing over all the ions 

 of any given class present, give resultant sums along the axes {h,)^ 

 {h)y {K)z for the ions of class (s). These three components are 

 equal and must be zero, since otherwise, in the transmission of light 

 close to the period of these ions, the influence would be predominant, 

 and, according to theory, there should be a marked rotation of plane- 

 polarised light near an absorption-band. 



As this is not so in general, 



In the presence of an external magnetic field, these components 

 cannot produce any rotation, since they are unchanged when resolved 

 along the rotating system of axes, and vanish by reason of the above 

 condition. This argument holds evidently only for diamagnetic 

 substances, which are not optically active ; and, in passing, it may 

 be noted, suggests a possible mode of attacking this last property. 



The magnetic rotation must then be caused by the fourth class of 

 components qj^ and we may ignore completely the presence of the 

 other components. The direction of rotation in qj is left-handed with 

 reference to the external field, and produces a magnetic force opposed 

 to it. These circuits are obviously the equivalent of the molecular 



T 



which may be considered as flowing in a separate circuit 



= [Kl = = 0. 



