THE EYE. 37 



* lens' there is the V H, or space filled with vitreous 

 humour, 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 accomplished 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 

 understand 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 /, 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 j^ou 

 thus form with stiff but very transparent jelly, you 

 would thus get a rough representation of the lens 

 of the eye. Now, in the ' lens' of the eye, our two 

 watch-glasees are represented by a very delicately 

 thin pliable membrane called the ' capsule' of the 

 lens, and so the whole ' lens' being firm, but pliable, 

 can be altered in shape by the 'ciliary muscle,' (Fig. 

 5, D 2) which is, as we have seen, attached around 

 its margin, so that when this muscle drags the lens 

 backwards against the stiff ' vitreous humour,' the 



