374 



FORMAL OPTICS. 



CHAPTER I. 



PRIMARY INDUCTION OF OPTICS. RAYS OF LIGHT 

 AND LAWS OF REFLECTION. 



IN speaking of the Ancient History of Physics, 

 we have already noticed that the optical philo- 

 sophers 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 men- 

 tioned, 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 and 

 the eye, all the rules of perspective follow directly. 

 The ancients practised this art, as we see in the 

 pictures which remain to us; and we learn from 

 Vitruvius 1 , that they also wrote upon it. Aga- 



1 De Arch. ix. Mont. i. 707. 



