WHAT DO ANIMALS SEE? I5I 



even if we did not know that their retinas consist mostly of 

 cones — and they have great visual acuity so that they can 

 see minute detail several times further away than we can. 

 Although their eyes are not telescopic in the sense that they 

 have a system of magnifying lenses they are telescopic in the 

 literal meaning of "seeing far". Sight is necessarily an im- 

 portant sense in animals that fly rapidly by day, and in 

 practically all birds it is the dominant sense. Hence the 

 comparatively enormous size of their eyes, in which most of 

 the globe is covered by skin and the part we see between 

 the eyelids is only a small window. The quickness of response 

 to the picture seen by the eye is also very important, for a 

 sparrow flying to a tree and alighting on a twig must be 

 equivalent to a jet pilot suddenly folding his plane's wings 

 and perching on a church tower. 



In night birds, such as owls and some others, as we have 

 seen, the picture is not nearly so clear, but there is a picture 

 of sorts in light so dim that day birds could see nothing. 

 Furthermore owls have little power of accommodation or 

 convergence — they cannot alter focus nor move their eyes 

 in their sockets, and for the latter reason they have to turn 

 their heads to see anything out of the direct line of vision. 

 They can turn their heads through more than i8o degrees 

 so that they can look more than directly backwards. Hence 

 the old Yankee yarn about how to catch an owl : if you find 

 one sitting on a post you have only to walk round it three 

 times and it will follow you round with its eyes until it 

 wrings its own neck ! 



The fishes, too, are not colour-blind and this again we 

 could infer from the part that bright colours play in the 

 mating displays of many of them that live in shallow water. 

 On the other hand, the colours of fishes that live in more 

 than very moderate depths of the sea are very different down 

 there from their appearance at the surface. Although water 

 is transparent it does absorb light, starting with the red end 

 of the spectrum, so that in descending below the surface the 

 light gets bluer as it fades out to the total darkness of great 

 depths. Consequently as soon as the red component in the 

 light is eliminated, red animals appear black because there 

 is no red light to be reflected from them : beyond the critical 



