FORMAL OPTICS. 



CHAPTER I. 



PRIMARY INDUCTION OF OPTICS. RAYS OF LIGHT AXD LAWS OF 



REFLECTION. 



IX speaking of the Ancient History of Physics, we have already 

 noticed that the optical philosophers of antiquity had satisfied 

 themselves that vision is performed in straight lines ; that they had 

 fixed their attention upon those straight lines, or rays, as the proper 

 object of the science ; they had ascertained that rays reflected from 

 a bright surface make the angle of reflection equal to the angle of 

 incidence and they had drawn several consequences from these 

 principles. 



We may add to the consequences already mentioned, the art of 

 perspective, which is merely a corollary from the doctrine of rectilinear 

 visual rays ; for if we suppose objects to be referred by such rays to a 

 plane interposed between them a.nd the eye, all the rules of perspec- 

 tive follow directly. The ancients practised this art, as we see in the 

 pictures which remain to us and we learn from Yitruvius, 1 that they 

 also wrote upon it. Agatharchus, who had been instructed by 

 Eschylus in the art of making decorations for the theatre, was the 

 first author on this subject, and Anaxagoras, who was a pupil of Aga- 

 tharchus, also wrote an Actinographia, or doctrine of drawing by 

 rays : but none of these treatises are come down to us. The moderns 

 re-invented the art in the flourishing times of the art of painting, that 

 i>, about the end of the fifteenth century ; and, belonging to that 

 period also, we have treatises' upon it. 



But these are only deductive applications of the most elementary 

 optical doctrines ; we must proceed to the inductions by which 

 further discoveries were made. 



1 De Arch. is. Mont. i. TOY. ' Gauricus, 1504. 



