SIGHT IN INVERTEBRATES 167 



when the light is bright they spread out and absorb the un- 

 wanted rays, but when it is dim they contract so that the 

 obKque Hght falhng on the sides of the tubes can pass 

 through. This enables the animal to see, but the picture it 

 gets is not nearly so sharp, and when the pigment cells are 

 fully contracted it must be very blurred. Nevertheless the 

 pigment cells are very quick in reacting and instantly adjust 

 themselves so that the image is kept as sharp as possible 

 while retaining no more than the necessary brightness. 



The compound eyes of many insects and Crustacea are 

 capable of colour vision. Experiments have shown that some 

 kinds of butterflies visit flowers of certain colours more than 

 others, particularly the blues and reds. The late Professor 

 Eltringham made another experiment which showed that 

 Small Tortoiseshell butterflies are sensitive to red. He painted 

 their eyes with a red dye which allowed only red light to 

 enter the eyes. If the butterflies were colour-blind to red 

 they would have been able to see nothing, but when they 

 were liberated they flew about with no apparent incon- 

 venience. It is a pity that he did not repeat his experiment 

 with other dyes to establish the range of colour vision enjoyed 

 by these insects. Later experiments have established that the 

 upper part of the compound eyes in many insects is sensitive 

 to colour but that the rest is colour-blind. In addition to 

 these experiments with live animals it has been found that 

 different colour-sensitive pigments can be extracted from 

 the compound eyes of both insects and crustaceans and that 

 they are associated with different types of light-sensitive 

 cells. 



Before leaving the subject of sight it is interesting to note 

 that very few animals have eyes that make use of another 

 method of forming an image, namely a pin-hole instead of 

 a lens. A small hole in a thin sheet of anything opaque forms 

 an image on a screen behind it, not by refraction as does a 

 glass lens, but by diffracdon. A pin-hole lens has no focal 

 length so that objects at all distances in front of it are in 

 focus on the screen behind ; the greater the distance between 

 pin-hole and screen the greater the magnification but the 

 lesser the brightness. The cup-shaped open eye of the limpet 

 is, as we have seen, probably incapable of forming an image, 



