340 Proceedings of Poyal Society of Edinburgh. [sess. 
It is clear that there are other parts of Dr Sang’s paper which 
might he greatly simplified by the use of an auxiliary angle ; hut it 
suffices to have shown the value of the method in the most compli- 
cated part of the investigation. 
[P.S.—^Nov. 23, 1891. — Mr B. T. Glazebrook has kindly given 
me a reference to Comptes Pendas, xeix. 538 (1884), where M. E. 
Bertrand has suggested the employment of glass prisms separated 
by a thin layer of Iceland spar.] 
