566 



SIGHT. 



than forty-eight degrees, pass through it, and, from both its den- 

 sity and figure, are considerably refracted towards the axis of the 

 eye, and on entering the aqueous humour they experience rather 

 a less degree of refraction. 



" Those rays which penetrate the pupil, and are received by 

 the lens, are still more refracted on account of the greater den- 

 sity of this medium. 



" The less density of the vitreous humour prevents the focus 

 of rays from being too short, and allows it to fall upon the retina 

 and exhibit the image of objects, though, from the laws of light, 

 necessarily inverted.'* 



By means of the indices of refraction above given, and the 

 known dimensions of the eye, it is easy to determine the path 

 which the rays of light travel through the humours of the eye, 

 whether they fall upon it in a parallel or a diverging condition. 



Let a b c be the eye, and d e an object at a considerable 



This figure shows how the images of external objects are formed on the retina. 



This figure shows how the rays of light issuing from a point fall divergingly on 

 the cornea, and are afterwards refracted so as to meet again in a single point on 

 the retina. 



