The raw materials are selected with great care to assure chemical purity, but whereas in most glasses the only impurities to be dreaded are those that are either infusible or produce a colouring effect upon the glass, for optical purposes the admixture of other glass-forming bodies than those which are intended to be present must be avoided on account of their effect in modifying the optical constants of the glass.
The general nature of the colouring ingredients employed, and the colour effects produced by them, have already been mentioned.
This colouring power is due to the presence of polychlorite, a substance whose chemical formula appears to be C4 8 1-160018, and which may be obtained by treating saffron with ether, and afterwards exhausting with water.
This crocin is a red colouring matter, and it is surmised that the red colour of the stigmas is due to this reaction taking place in nature.
The explanation of animal (and vegetable) mechanism, colouring, habits, &c., as advantageous to the species or to its ancestors, are only gradually being carried further.