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CHAPTER II. 



EARLIEST STAGES OF OPTICS. 



THE progress made by the ancients in Optics 

 was nearly 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 Reflection 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 carried. 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 



