2o8 PHYSIOLOGY FOR BEGINNERS CHAP. 



magnifying power. The eye still contains the vitreous humour, 

 a clear colourless jelly-like material. Before disturbing the 

 vitreous humour look through it on to the back of the eye ; 

 what you see is the greyish-yellow retina with a few of its larger 

 blood-vessels. Turn out the vitreous humour and notice that 

 the retina is a delicate membrane, very easily torn. When 

 it is removed the choroid which is left behind appears darkly 

 pigmented in some parts and iridescent (glistening with many 

 colours) in others. This iridescence is peculiar to certain 

 animals ; it is present in the bullock's eye, but not in that of 

 man. Lift up the choroid and notice that it is loosely attached 

 to the tough outer coat, the sclerotic. 



The Formation of Clear Images on the Retina. 

 The eye as the organ of sight has for its object the reception 



FIG. 98. The formation of an image on the retina. 



of light in such a way as to excite in the optic nerve impulses 

 which, reaching the brain, produce the sensation of sight. The 

 retina, a membrane of elaborate structure, contains the ter- 

 minations of the optic nerve spread out in the form of a curved 

 sheet. All other parts of the eye are either for the purpose 

 of bringing the rays of light to the retina in such a way as to 

 form on it clear images of the things we see or of protecting 

 or nourishing the retina. 



If a convex lens be fixed a few feet in front of a candle flame 

 in an otherwise dark room, and a sheet of pape^fte held on the 

 other side of the lens so as to form a screen, it will be found 

 that at a certain distance of the paper from the lens a clear 

 inverted image of the candle flame will be seen on the sheet of 

 paper. The rays from the candle are then said to be brought 

 to a focus on the screen. In the eye, images are formed in a 



