178 H. C, SORBY. 
study to other substances will lead to a great increase of our 
knowledge ‘of the relation between optical and chemical 
properties, and throw much light on many interesting ques- 
tions connected with molecular physics and other branches 
of science ; and it is this conviction which has made me 
anxious to give a complete description of the spectra of a 
substance which exhibits them to very great advantage. 
