406 ELEMENTARY PHYSIOLOGY less. 



If the lens were made of some extensible, elastic sub- 

 stance, like india-rubber, pulling it at the circumference 

 would render it flatter, and thereby lengthen its focus ; 

 while, when let go again, it would become more convex, 

 and of shorter focus. 



Any material more refractive than the medium in which 

 it is placed, if it have a convex surface, causes the rays of 

 light which pass through the less refractive medium to 

 that surface to converge towards a focus. If a watch-glass 

 be fitted into one side of a box, and the box be then filled 

 with water, a candle may be placed at such a distance 

 outside the watch-glass that an image of its flame shall 

 fall on the opposite wall of the box. If, under these cir- 

 cumstances, a doubly convex lens of glass were introduced 

 into the water in the path of the rays, it would act (though 

 less powerfully than if it were in air) in bringing the rays 

 more quickly to a focus, because glass refracts light more 

 strongly than water does. 



A camera obscura is a box, into one side of which a lens 

 is fitted, so as to be able to slide backwards -ind forwards, 

 and thus throw on the screen at the back of the box dis- 

 tinct images of bodies at various distances oflf. Hence 

 the arrangement just described might be termed a water 

 camera. 



The eyeball, the most important constituents of which 

 have now been described, is, in principle, a camei'a of the 

 kind described above — a water camera. That is to say, 

 the sclerotic answers to the box, the cornea to the watch- 

 glass, the aqueous and vitreous humours to the water 

 filling the box, the crystalline to the glass lens, the 

 introduction of which was imagined. The back of the 

 box corresponds with the retina. 



But further, in an ordinary camera obscura, it is found 

 desirable to have what is termed a diaphragm (that is, 

 an opaque plate with a hole in its centre) in the 

 path of the rays, for the purpose of moderating the litrht 

 and cutting oft' the marginal rays which, owing to certain 



