164 THE HUMAN BODY. 



refracted by the cornea; it is the same if we look at a 

 distant object, the rays from which are less divergent; if this 

 object becomes more luminous, or if we approach it, the 

 pupil contracts in proportion. 



Optic centre, visual angle, appreciation of the size of objects. 



The rays emanating from two points of an object, p H (fig. 



38), converge toward the optic centre o, at a point in the eye 



a little behind the crystalline, and thus form an angle P o H, 



which is called the visual angle. From the optic centre these 



rays diverge to the retina, and form an angle, / o //, equal to 

 the first, the base of which, corresponding to the retina, 

 measures the size of the image which they form upon it. The 

 visual angle therefore gives us an idea of the size of objects, 

 and enables us to compare them, but in order that our ideas 

 may be exact, they must be confirmed by our notions of dis- 

 tance. In fact, several objects of unequal size, P H, P'H', 

 P"H", may be placed at such distances, ABC (fig. 38), as to 

 subtend the same visual angle; we must therefore estimate 

 their relative distance, in order to judge correctly of their 

 size. We can also obtain a knowledge of their size, if we know 

 that of any portion of the object which we see, or the size 

 of another object placed at an equal distance. Thus, when 

 we look at a ship at sea, we can judge of its size by that of 

 the men whom we see upon it; the height of a balustrade 

 enables us to calculate approximately that of the building of 

 which it forms a part. When the means of comparison fail 



