﻿912 Messrs. Roberts, Smith, and Richardson on 



It is evident that the Schwarzschild constant has a very 

 definite physical significance, but what this may be it is not 

 possible to say. From the values obtained with the three 

 types of plate it is evident that it is not greatly dependent 

 on either the size of the grain or on whatever factors ulti- 

 mately determine the sensitivity of a plate to light, but a 

 fuller knowledge of the composition of the silver halide- 

 gelatine emulsion and of its physical properties would seem 

 to be necessary before a complete explanation can be arrived at. 



This investigation was undertaken at the suggestion of 

 Prof. T. R. Merton, F.R.S., to whom I am deeply indebted 

 both for the loan of almost all the apparatus used, and for 

 much valuable help in the course of the investigation. 

 Thanks are also due to Mr. 0. H. Bosanquet for several 

 valuable suggestions. 



Balliol College Laboratory, 

 Oxford. 



LXXVII. Magnetic Rotatory Dispersion of certain Paramag- 

 netic Solutions. By R. W. Roberts, M.Sc, J. H. Smith, 



M.Sc, and S. S. Richardson, D.Sc, A.R.C.Sc* 



ONE of the anomalies met with in the examination of the 

 Faraday effect is that, whereas the salts of iron in 

 solution give rise to a negative rotation, those of the strongly 

 magnetic elements, nickel and cobalt, produce a rotation of 

 the plane of polarization in the opposite sense — that is, in the 

 direction of the rotation produced by the great majority 

 of compounds both inorganic and organic. In the case of 

 cobalt sulphate the rotation, though positive, is almost zero. 

 It is noteworthy, however, that the measurements on which 

 such statements are based have been made only with refer- 

 ence to the D line or at most a few lines in the visible 

 spectrum, and some years ago it occurred to one of us that 

 further information might be obtained by examining the 

 course of the rotatory dispersion in the ultraviolet. Experi- 

 ments were carried out in 1916 on CoS0 4 , and the spectrum 

 photographs showed that the rotation of this salt becomes 

 strongly negative in the ultraviolet. The principal absorp- 

 tion-band lies on the borders of the visible and ultraviolet, 

 and the rotation, which has a small positive value up to the 

 edge of the band in the visible spectrum, becomes negative 

 where the spectrum reappears, and remains negative through- 

 out the ultraviolet up to the point where general absorption 

 * Communicated by Prof. L. R. Wilberforce, M.A. 



