108 PHYSICAL SCIENCES IN ANCIENT GREECE. 



the proper colour of each object ; for some things, 

 as the heads, and scales, and eyes of fish, are seen 

 in the dark ; but then they are not seen with their 

 proper colour 1 . 



In all this there is no steady adherence either 

 to one notion, or to one class of facts. The dis- 

 tinction of Power and Act is introduced to modify 

 the Idea of Transparency, according to the formula 

 of the school; then Colour is made to be some- 

 thing unknown in addition to Visibility ; and the 

 distinction of " proper" and " improper" colours is 

 assumed, as sufficient to account for a phenomenon. 

 Such classifications have in them nothing of which 

 the mind can take steady hold; nor is it difficult 

 to see that they do not come under those condi- 

 tions of successful physical speculation, which we 

 have laid down (F). 



1 De Anim. ii. 6. 



