210 



ELEMENTARY PHYISOLOGY 



to a handle. Fig. 197 represents the same apparatus in section. If we cover 

 the box with a black cloth, we convert it into a ' camera obscura,' that is, a 

 dark chamber in which the images of outside objects may, with the aid of the 

 lens, be caught on a screen. If we fill the box with water, the convex surface 

 of the liquid in contact with the watch-glass itself acts as a convex lens, thus 

 producing an image without the use of the latter. The student will now 

 notice that the apparatus just described represents the various important 

 structures of the eye. The wood or metal forming the greater part of the box 

 represents the sclerotic ; the black lining, the choroid ; the watch-glass, the 



Fig. 197. 



cornea ; and the ground-glass screen, the retina. The diaphragms are to take 

 the place of the iris ; and the magnifying-glass that of the crystalline lens. 

 The water in the box represents the humours of the eye ; and by using a 

 transparent material for the back, we are enabled to see the images which fall 

 on the ground-glass screen. 



The following experiments are to be performed in a darkened room : 

 Place a lighted candle a few feet in front of the watch-glass, and on a level 

 with its centre. Now move the ground-glass screen to and fro between the 

 watch-glass and the back of the box till a distinct inverted image of the candle 

 is thrown on it. The rays of light from the candle, on reaching the watch- 

 glass, are refracted or bent, producing an image (fig. 198). The water is the 



Fig. 198. 



refracting medium in this case, and, having a convex surface in contact with 

 the watch-glass, it acts like a convex lens. Thus we have illustrated the use 

 of the transparent cornea of the human eye, and also of the watery fluid 

 behind it. 



Now repeat this experiment, first placing a convex lens behind the watch- 

 glass, and notice that the image is formed at a shorter distance than when the 

 water is the only refracting medium (fig. 199). In the same manner the 

 crystalline lens assists in the formation of images in the eye, and helps to 

 bring the images forward to such an extent that they are clearly defined on 

 the retina. 



