2 1 2 The Structure of Coloured Bodies. 



nature of ultra-microscopic structures generally would be 

 a simple matter. Such is, however, not the case. Though, 

 as we shall see, structure does commonly manifest colour 

 I)hcnomena, in numbers of instances no such effect is ap- 

 parent. In other instances colour is produced giving rise to 

 appearances of the most distinctive kind, but which, owing 

 to our ignorance of the cause of such colour-phenomena, have 

 for us no structural significance. So-called absorption colours 

 are of this nature, since, though many attempts have been 

 made to explain their production on a physical basis, they 

 have, so far as I am aware, as yet had no satisfactory 

 explanation. 



Colour alone, therefore, not being a universal manifesta- 

 tion of ultra-microscopic structure, we must in such instances 

 rely on other optical phenomena as indications of structure. 

 Such are polarisation, reflection, opalescence, &c. These I 

 shall subsequently consider and endeavour to show to what 

 extent they are indicative of structure. 



