14^ THE SENSES OF ANIMALS 



Another defect in all simple lenses is due to white light 

 being a mixture of different wave-lengths. The light of each 

 wave-length is focused a little in front of or behind that of 

 the others, so that only one of the overlapping images is in 

 sharp focus. As a result the image is slightly blurred and the 

 outlines of objects are edged with coloured fringes, especially 

 with red and purple which have the longest and shortest 

 wave-lengths. Here again the use of a diaphragm or an iris 

 so that only the centre of the lens is used decreases the 

 "chromatic aberration". 



In day animals the variation in size of the pupil is not 

 nearly as great as in nocturnal ones where it opens very 

 wide in poor light. In nocturnal animals the pupil when 

 contracted is often a vertical slit, as in the cat, fox and 

 adder, but there is no general rule and in many others it 

 stays circular and may shut down to no more than a pin- 

 hole during the day. The larger the pupil when open the 

 more difficult it is to shut it down to a pin-hole ; closing to 

 a slit probably makes a neater job and avoids puckering 

 the edge, although it cuts out some of the width of the field 

 of vision. A vertical pupil is found in animals, generally 

 predators, that look more or less ahead to concentrate on 

 their prey; their potential victims, such as the hoofed 

 animals, on the other hand, which need to keep a sharp 

 look-out over a wide field, have horizontal pupils. 



In birds the pupil can be closed, but it is nearly always 

 kept fully open, even in bright light, because birds have 

 another way of cutting out the blurred image produced by 

 the edge of the lens, as we shall soon see. Penguins are an 

 exception, for their pupils are very small in bright light and 

 in some kinds they pucker into a square or many-sided 

 shape ; no doubt the iris opens widely when the penguin is 

 under water in dim light. The iris is often brightly coloured 

 in birds, reptiles, amphibians and fishes, but in the mammals 

 it is duller, usually some shade of brown or yellow. In birds 

 with brightly coloured irises the pupil is sometimes opened 

 and shut rapidly as an expression of emotion, or to accen- 

 tuate the eyes as a threatening display in defence or aggres- 

 sion. 



The image made by the lens falls on a screen at the back 



