IN QUARTZ OF LIGHT IN THE VISIBLE REGION OF THE SPECTRUM. 
2‘t7 
( 3 ) By taking into account the infra-red absorption it has been possible to calculate 
the rotations in degrees per millhnetre with an average error not exceeding O'OOIS 
degrees per millimetre. 
(4) Wiedemann’s Law of the proportionality of natural and magnetic rotations 
over a range of wave-lengths has been verified in the case of quartz l)ut does not hold 
good in the case of optically active liquids. 
A large part of the cost of the investigation has been defrayed by grants from the 
Besearch Fund of the Government Grant Committee of the Royal Society ; the author 
is indebted to this Committee for generous consideration during a period of seven 
years. He is also indebted to Prof. Armstrong for the interest he has taken in the 
investigation during the whole of its course, to Prof. Sir William Tilden for lielp 
in securing the loan of a valuable polarimeter, to Mr. F. Twyman of Messrs. A. Hilger 
and Co. for many useful suggestions, to Mr. T. W. Dickson for a mathematical 
investigation of the experimental figures, and to Mr. W. P. Paddison and Mr. H. R. 
CouRTMAN for help in the arduous work of reading the polarimeter under difficult and 
tedious conditions. 
2 Q 
VOL. CCXII.—A. 
