xix IIII. KYI. AND THK SENSE UK SH1HT 209 



similar way. The retina is the screen, and inverted images 

 of external objects are formed on it by the crystalline lens, 

 which, however, is helped by the curvature of the cornea ; 

 since the eyeball is filled with the aqueous and vitreous humour 

 the convex surface of the cornea acts as a convex lens in the 

 same way as a glass box with a convex front will act as a 

 convex lens when filled with water. If, when a sharp image has 

 been obtained in the simple experiment with the glass lens, the 

 candle be moved nearer the lens, its image on the paper becomes 

 indistinct, but a clear image is obtained by shifting the paper 

 farther from the lens. On the other hand, if the candle be moved 

 farther from the lens, the image becomes again indistinct, but 

 may be made once more clear by moving the paper nearer the 

 lens. If, however, the distance of the paper from the lens is 

 kept the same after the candle has been shifted, a clear image 

 of the candle can only be obtained by using a stronger lens 

 in the first case and a weaker one in the second case. Thus 

 we see that in order to bring the rays from a near object to a 

 focus as soon as, that is, at the same place as, the rays from a 

 distant object, a stronger lens, that is, a more sharply curved lens, 

 must be used. The images formed on the screen are inverted 

 images, and so are those on the retina ; and such an inverted 

 image may be seen on the retina of a bullock's eye by removing 

 the hinder part of the sclerotic and choroid, and looking at the 

 back of the retina when the eye is directed to a candle flame. 

 Hence in a certain sense we may be said to see things upside 

 down ; but we see everything in this way, not some things 

 only, and our sensations do not tell us that the images are 

 inverted ; we only learn this by examining other eyes. When 

 the image of an object falls on the lower part of the retina, we 

 do not know from the sensation that the lower part of the 

 retina is being affected ; we have a sensation of an object lying 

 high up, so that we have to raise our hand if we try to touch it. 

 Similarly a spot of light, rays from which affect the upper part 

 of the retina, sams to us low down ; one affecting the inner 

 side, the nose side of the right eye, for instance, seems to be 

 towards our right hand ; one affecting the outer side, the cheek 

 side of the same eye, seems to be on our left hand, and so on. 

 \Vc " refer," as it is said, every sensation started in the retina 

 to some or other part of the external world ; we never think 



