720 



Professor Andrew Gray 



[April 29, 



of axes, tlie force required for the acceleration towards the centre, 

 and which is applied to it by the medium, is greater or less according 

 as the direction in which the molecule is carried round is with or 

 against the direction of rotation of the gyrostats. That is, the effect 

 of the rotation is to virtually increase the inertia of the molecule in 

 the one case and diminish it in the other. 



These molecules embedded in the medium are supposed to be 

 exceedingly small, and to be so distributed that the medium may, in 

 the consideration of light propagation, be regarded as of uniform 

 quality. Lord Kelvin's last form of molecule, it may be pointed out, 

 if the surface of its sheath adheres to the medium, will have efficiency 

 as an ordinary single gyrostat as regards rotations of the molecule, 



Fig. 16.— Path of the Bob of a Gyrostatic Pendulum. 



As the pendulum moves, it passes from one ray to another on the 

 opposite side, and the direction of motion at each swing alters through 

 the angle between two rays. The central parts of the rays are left out. 

 The marking point does not pass exactly through the centre. 



and efficiency likewise as regards translational motion of the centre 

 of the molecule. The former efficiency can be made as small as may 

 be desired by making the molecule sufficiently small ; the latter may 

 be maintained at the same value under certain conditions, however 

 small the molecule be made. 



The lately discovered effect of a magnetic field in giving one 

 period of circular oscillation of a particle or another according as the 

 particle is revolving in one direction or the other about the direction 

 of the magnetic force, is connected with magneto-optic rotation. 

 There is a connection between velocity of propagation and frequency 

 of vibration, which is exemplified by the phenomena of dispersion. 

 In the Faraday effect, the two modes of vibration, if of the same period, 

 have different velocities of vibration, consequently these two modes 



