OF SIGHT. 51 



retina will be inverted. We may satisfy ourselves of this 

 by direct observation. The eye of the white rabbit being 

 destitute of the black pigment of the choroid, is quite trans- 

 parent. Take the eye, soon after the death of the animal, 

 and arrange it in one end of a tube, so that the cornea will 

 face outwards ; then if we look in at the other end of 

 the tube, we may see objects to which it is directed exactly 

 pictured upon the retina, but in a reversed position. 



79. The mechanical structure of the eye may be per- 

 fectly imitated by art. Indeed, the camera obscura is an 

 instrument constructed on the very same plan. By it, exter- 

 nal objects are pictured upon a screen, placed at the bottom 

 of the instrument, behind a magnifying lens. The screen 

 represents the retina ; the dark walls of the instrument 

 represent the choroid ; and the cornea, the crystalline lens 

 and the vitreous humor combined, are represented by the 

 magnifying lens. But there is this important difference, that 

 the eye has the power of changing its form, and of adapt- 

 ing itself so as to discern with equal precision very remote, 

 as well as very near, objects. 



80. By means of muscles which are attached to the ball, 

 the eyes may be rolled in every direction, so as to view ob- 

 jects on all sides, without moving the head. The eyes are 

 usually protected by lids, which are two in the mammals, 

 and generally furnished with a range of hairs at their edges, 

 called eye-lashes. Birds have a third lid, which is vertical ; 

 this is also found in most of the reptiles and a few mam- 

 mals. In fishes, the lids are wanting, or immovable. 



81. The eye constructed as above described is called a 

 simple eye, and belongs more especially to the vertebrate 

 animals. In man, it arrives at its highest perfection. In 

 him, the eye also performs a more exalted office than mere 

 vision. It is a mirror, in which the inner man is reflected. 

 His passions, his joys, and his sorrows, his inmost self, are 



