OF NATURAL PHILOSOPHY. 189 



which permits a bluish-green light to pass through 

 it, is a frontier instance between the transparency 

 of pellucid bodies and the opacity of metals, and it 

 prevents a breach of the law of continuity between 

 transparent and opake bodies, by exhibiting a body 

 of the class generally regarded the most opake in 

 nature, as still possessed of some slight degree of 

 transparency. It thus proves that the quality of 

 opacity is not a contrary or antagonist quality to 

 that of transparency, but only its extreme lowest 

 degree. 



