220 



From fig. 2 the very remarkable shape of the dlspersion-fiirves 

 may be seen, which al a wave-length of about 5800 A. U. show 

 a maximum. For wave-lengths smaller than 5800 A. U. tlie rotation 

 of the plane of polarisation increases with increasing wave-length, while 

 for those greater than 5800 A. U. it diminishes with increasing 



