358 GREEK SCIENCE. 



we learn, that the first book treated of the relations between light and 

 vision, of their resemblance and of their difference. It was probably a 

 philosophical dissertation after the manner of Aristotle. Ptolemy 

 supposes that vision is effected by means of a pyramid of visual rays, 

 of which the vertex is at the eye, and the base at the object seen. This 

 agrees with the notions of Euclid : some writers, earlier than either of 

 these, taught, as the moderns do, that the rays of light proceed from 

 the visible object ; but this notion had few partizans, while the notions 

 of Euclid and Ptolemy prevailed extensively. Vision by the axis of 

 the pyramid is, according to Ptolemy, more correct and perfect than 

 that by oblique rays. Vision makes bodies known, reveals their mag- 

 nitude, colour, figure, rest, and motion ; but all this requires light. 



Shadows are not seen : we know them only by privation. 



We can see better with two eyes than with one : with one only we 

 do not see the object precisely at the same place as with two. We 

 see the object simple, if the two axes of the pyramids are directed in 

 the same manner upon the object ; we see the same object double, if 

 the axes are not directed naturally, and if the distance is a little less 

 than that between the two eyes. 



Colour makes part of bodies, it is the exterior crust. The eye per- 

 ceives the direction of the visual ray which it sends towards the body ; 

 it perceives, in like manner, the length ; it judges of the magnitude of 

 the object, from the length of the pyramid combined with the mag- 

 nitude of its base. If the humidity of the visual ray be promptly dis- 

 sipated, bodies are seen better when near ; if it be slowly dissipated, 

 they are seen better at a distance. 



That which causes certain persons to see better than others, is the 

 abundance of the visual virtue ; which, like all other faculties, fails in 

 old men. 



The moon has a colour which is peculiar to it, and which is only 

 perceived in eclipses. 



When we have long contemplated an object highly coloured, and 

 then direct our eyes to another object, we attribute to that the colour 

 of the former. 



Things which we see by reflection partake of the colour of the 

 mirror ; as those which we see through a diaphanous, or transparent 

 body, assume its colour. 



When we observe a fire or a light at the horizon, beyond a pool of 

 water, we perceive a long luminous train which follows our motion. 



A sail seen from far, appears more curved than it is in fact ; because 

 the middle, which is seen directly, is perceived better than the edges, 

 which appear to fly. Thus painters, when they would excite the idea 

 of anything concave, give a less vivid tint to the middle than to the 

 edges ; and the contrary, if they would give the idea of convexity. 



Similar to these are the remaining speculations in the second book. 



In the third book Ptolemy proceeds to the subject of mirrors. The 

 principal propositions are these : In the plane mirror, the object is 



