238 Mr. S. S. Richardson on Magnetic Rotary 



For substances which are transparent in the visible and 

 near infra-red, and for which the infra-red values of z are 

 negligible, X l5 X 2 • • • will represent ultraviolet periods only. 

 If X 2 is considerably less than X b then since bi = a{k~?, 

 Z> 2 = a 2 X 2 2 , the ratio b 2 /b 1 and therefore h 2 \k x will be much 

 smaller than a 2 \a x . Further, the ratio 

 X 2 X 2 



x 2 -V : x 2 -x 2 2 



(if X>XJ will be less than its square. The effects of two 

 free-periods are therefore more sharply differentiated in 

 magnetic rotary dispersion than in ordinary dispersion. 

 Further, the resonators which give rise to the constant term 

 a in (3) have no appreciable effect on magnetic rotation, 

 and these therefore do not give rise to the term &/X 2 in the 

 expression for 8 as is sometimes stated *. Hence the values 

 of X b X 2 . . . in (8 ) are those of the dispersional periods only 

 in the ultraviolet. If there is only one such period we have 



IE now this equation is used to determine X x from a series 

 of values of X, say approximately equally spaced along the 

 spectrum, the values so obtained should agree within the 

 limits of experimental error with the value of \ x obtained 

 from natural dispersion if there is only one free-period. 

 Want of such agreement, particularly if accompanied by a 

 progression in the calculated values of X 1? indicates the 

 existence of at least two free-periods. In the latter case, 

 and where sufficient progression exists, a closer approxi- 

 mation to Xx may be obtained by neglecting the dispersional 

 part of 



whilst retaining its constant part. Equation (8) may then 

 be written 



*= k (w^f +k * ■ ■ ■ ■ w 



and Xi, /t'x, k 2 deduced from three values of </>. 



In the experiments recorded below I have carried the 

 measurements of magnetic rotation as far into the ultra- 

 violet as the nature of the liquids used and the apparatus at 

 my disposal would allow, as it seemed probable that an 

 extended dispersion curve would give in most cases more 

 information concerning the dispersional-periods than one 

 confined to the visible spectrum. 



* Cf. Wood's ' Physical Optics,' p. 530. 



