100 PHYSICAL SCIENCES IN ANCIENT GREECE, 



CHAPTER II. 



EARLIEST STAGES OF OPTICS. 



THE progress made by the ancients in Optics was nearlj proportional 

 to that which they made in Statics. As they discovered the true 

 grounds of the doctrine of Equilibrium, without obtaining any sound 

 principles concerning Motion, so they discovered the law of the Reflec- 

 tion of light, but had none but the most indistinct notions concerning 

 Refraction. 



The extent of the principles which they really possessed is easily 

 stated. They knew that vision is performed by rays which proceed in 

 straight lines, and that these rays are reflected by certain surfaces 

 (mirrors) in such manner that the angles which they make with the 

 surface on each side are equal. They drew various conclusions from 

 these premises by the aid of geometry ; as, for instance, the convergence 

 of rays which fall on a concave speculum. 



It may be observed that the Idea which is here introduced, is that 

 of visual rays, or lines along which vision is produced and light car- 

 ried. This idea once clearly apprehended, it was not difficult to show 

 that these lines are straight lines, both in the case of light and of sight. 

 In the beginning 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 cer- 

 tain 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 car- 

 ried 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 instance), 

 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. 



