152 
Proceedings of the Royal Society of Edinburgh. [Sess. 
No attempt can yet be made to interpret the results in the light of 
chemical constitution. The preparation of compounds for use as pigments 
may produce complex conditions of which we are unaware, and it will be 
necessary for comparison to determine the diffuse reflective powers of the 
same compounds in their chemically pure state. It is in any case unlikely 
that the problem of the interpretation of reflection spectra will be any 
simpler than that of absorption spectra, and the exhaustive investigation 
of numerous series of compounds will probably be required before any 
satisfactory theory can be suggested which will fit the observed facts. 
I am indebted to Dr R. A. Houstoun for advice during the progress of 
the foregoing investigation, which was carried through during my tenure 
of the Houldsworth Research Studentship in the University of Glasgow. 
{Issued separately May 21 , 1915 .) 
