106 PHYSICAL SCIENCES IN ANCIENT GREECE. 



of Euclid's " Treatise on Optics," some of the argu- 

 ments are mentioned by which this was established. 

 We are told in the Proem, "In explaining what 

 concerns the sight, he adduced certain arguments 

 from which he inferred that all light is carried 

 in straight lines. The greatest proof of this is 

 shadows, and the bright spots which are produced 

 by light coming through windows and cracks, and 

 which could not be, except the rays of the sun 

 were carried in straight lines. So in fires, the 

 shadows are greater than the bodies if the fire be 

 small, but less than the bodies if the fire be greater." 

 A clear comprehension of the principle would lead 

 to the perception of innumerable proofs of its truth 

 on every side. 



The Law of Equality of Angles of Incidence and 

 Reflection was not quite so easy to verify ; but the 

 exact resemblance of the object and its image in a 

 plane mirror, (as the surface of still water, for in- 

 stance,) which is a consequence of this law, would 

 afford convincing evidence of its truth in that case, 

 and would be confirmed by the examination of 

 other cases. 



With these true principles was mixed much 

 error and indistinctness, even in the best writers. 

 Euclid, and the Platonists, maintained that vision 

 is exercised by rays proceeding from the eye, not 

 to it ; so that when we see objects, we learn their 

 form as a blind man would do, by feeling it out 

 with his staff. This mistake, however, though Mon- 



