340 Transactions of the Boyal Society of South Africa. 
light was successfully accomplished. The plate was attached to an iron 
lid, which could be rotated while a Bunsen burner was playing on it. 
The effect of heat on the birefringence of corundum was found to be 
exceedingly small — in fact, such as might be caused by the mere expan- 
sion of the mineral. The spectrum of the ruby depends, therefore, on the 
chromium, and not on the crystalline structure of the alumina ; in fact, 
apparently the only possible explanation left is the nature of the chromium 
atom itself. 
The author wishes to acknowledge the most valuable help and sugges- 
tions given by Mr. Max Weber, curator of the Johannesburg Museum, in 
this work. 
