WHAT DO ANIMALS SEE? I49 



all, for the sensitivity of its retina is a hundred times greater 

 than ours. In spite of this the owl's powers of night vision do 

 no more than enable it to find its way and avoid colliding 

 with things ; it certainly does not hunt its prey by sight alone. 

 As we shall see in a later chapter, the owl catches a mouse on 

 a dark night by using its extraordinarily efficient sense of 

 hearing. 



Accommodation has another use besides focusing the lens 

 to give a sharp picture ; the amount of tension produced by 

 the muscles in accommodating helps in judging the distance 

 of an object. This comes about not through any fixed scale 

 of tensions but is learned through experience of focusing on 

 objects near and far. Judgement of distance is also helped by 

 the amount of convergence necessary in examining an object, 

 and this, too, has to be learnt by experience. When an 

 animal with forwardly directed eyes looks at any object it 

 adjusts the eyes so that the part of the image required falls 

 on the fovea of each. Consequently the nearer the object the 

 more the eyes have to be turned inwards towards each other 

 until, when it is very close, there is a distinct squint. 



Judging distance by these means, however, is not the only 

 thing about using the eyes that has to be learnt by experience. 

 The eyes of a newly-born animal are little more use to it than 

 the light-sensitive skin of the earthworm, they enable it to 

 distinguish light from darkness but the images falling on the 

 retina mean little or nothing to it — how could they, for it 

 has never seen anything before? By exploring its surround- 

 ings mainly by touch and smell as well as by sight, it 

 gradually learns the physical properties of its environment 

 and co-ordinates what it is told by the different senses so that 

 it learns the meaning of the images focused on the backs of 

 its eyes. Eyes, without experience, are of comparatively little 

 use for telling animals what is going on around them. 



Sight is one of the most acute of our senses and conse- 

 quently we attach great importance to it, but even our eyes 

 are most fallible organs and are very easily deceived, as is 

 well shown by the many optical illusions that are made to 

 trick the eyes for amusement or instruction. We betray the 

 unreliability of our sight if anyone shows us something we 

 have never seen before. "Whatever is that !" we exclaim, 



