1 IS I J A L J I DGMKNTS 



295 



stereoscope. This instrument consists of two prisms or half 

 lenses placed, with the thin edge inwards, about the same distance 

 apart as the eyes. The dissimilar pictures P^ and Pg ^^^ fixed 

 one in front of each lens, and a median screen cuts off each opposite 

 picture. By means of this apparatus we obtain a single picture 

 in the most pronounced relief, and situated apparently at P. 

 (Fig. 79). 



6. The Visual Field. When one eye is closed and the other fixed 

 on a certain point the whole range of objects which can be seen 

 without moving the eye or the head is called the visual field. The 

 angle subtended by these objects is called the visual angle. Witli 

 both eyes opened and fixed we command 

 a greater range. — :> — 



We may summarise the advantages of / ', 



binocular vision as follows : < '> 



(i.) The visual field is increased. ,' * 



(ii.) W^e receive impressions of solidity. ,' \ 



(iii.) We have a most important means p 

 of judging distances. ~ 



(iv.) The loss of one eye does not reduce 

 us to blindness. 



./ n 



w 



Fig. 79. — Diagram of stereoscope. 



Analysis of Retinal Stimuli. 



We have examined the eye as an optical 

 instrument and discussed the mechanism 

 whereby an inverted image of the external 

 object is formed on the retina. We have 

 also indicated that this image may be 

 imprinted on the retina photoehcmically— 

 a rapidly reversible reaction. It may 

 puzzle the student as to why, the image 

 being inverted on the retina, we see things right side up. 

 Seeing is an acquired reaction, i.e. it implies learning. We do not 

 see the image on the retina. What we do get is a series of nerve 

 impulses coming to the cortex from one or more of our seven 

 million cones. Just as with the other receptors, primary analysis 

 takes place in the retina — number and position of cones stimulated, 

 rate of bleaching of visual purple, etc. — and these changes, in the 

 light of previous experiences, are analysed and interpreted in the 

 cortex. 



Purkinje Figures. Certain parts of our retina may be seen by 

 ourselves, as shadows it is true, but nevertheless objectively. 

 These are the blood-vessels and certain bundles of nerve-fibres. 

 If, in a dark room, the eye is kept motionless, and light from a 



