44 LESSONS IN HOKSE JUDGING. 



back of your other hand. Now move the lens 

 gentlj to and fro, and you will see a beautiful 

 little image of the window frame, gas jet, or 

 candle-light (whichever you are using) on the 

 back of your hand. Now you have got this per- 

 fect image by moving the lens backwards and 

 forwards between your hand and the light, and 

 you will have found that correct distance is 

 everything; that is to say, had you held the lens 

 an inch nearer or an inch further off, you would 

 not have got a sharp, clear image. Now look at 

 Fig. 5, Z), and you will see that behind the ^ lens ' 

 there is the V H, or space filled with vitreous 

 humor, and in front of it there is the 'iris,' so 

 that it is quite evident that the ' lens ' of the eye 

 cannot be moved backwards and forwards, an 

 inch forward now, an inch backward then, as 

 you have done in your experiment, because the 

 whole eye is only about an inch from front to 

 back, so that the focusing of the image on the 

 'retina' by the 'lens' must be accomphshed in 

 another way altogether, and in this way the 

 shape of the ' lens ' itself is altered. 



29. — We must say a few words about the con- 

 struction of the ' lens ' of the eye, or you will not 

 luiderstand what is meant by a cataract, so that 

 after we have seen how the ' lens ' is made we can 

 see how it alters its shape in focusing. Turning 

 to the diagram Fig. 5, D I, you see that the lens 

 of the eye can be quite well represented by placing 

 two ordinary old-fashioned watch-glasses together 

 at their edges. Now, if you could fill the cavity 

 you thus form with stiff but very transparent 



