SENSES, INSTINCTS, AND INTELLIGENCE 325 



cesses on its surface. Franz regards it as an intra-ocular 

 sense-organ, serving for the perception of the fluctuations 

 of pressure in the vitreous humour which arise from the 

 movements of the lens in accommodation. 



Little seems to be known in regard to the colour-sense 

 of birds, and experiments are not very easy. Thus, if 

 certain birds do not pick up seeds coloured blue, it does not 

 follow that they do not see them ; it may simply mean that 

 they have formed no association between a blue object and 

 food. Erna Hahn (19 16) found that some parrots, e.g. 

 Melopsittacus and Cyanospiza, seemed unable to become 

 accustomed to take their food on an area illumined by 

 coloured light. Fowls, on the other hand, picked up blue 

 seeds and uncoloured seeds in a blue area ; they made no 

 use of those in ultra-blue or ultra-violet areas. 



The use of brightly coloured pods and flowers by Bower- 

 birds suggests a colour-sense, and so do the bright colours 

 of many male birds, but it is always difficult to distinguish 

 what may be discrimination of colour from what may be 

 only discrimination of differences in the intensity of the 

 light reflected from the surface. 



The experiments of Iless on fowls, pigeons, and birds 

 of prey show that birds' vision and man's are practically 

 the same towards the orange-yellow end of the spectrum 

 (long wave-lengths), but that birds' vision falls far short 

 of man's towards the blue-violet end. In fact, birds 

 cannot see blues and violets. Hilzheimer notes that our 

 vision would be like a bird's, as regards colour-perception, 

 if we wore reddish-yellow glasses. 



The retina or sensitive surface shows the usual nerve- 

 endings, known as the rods and cones, but the cones are 

 relatively few in number. Cones are believed to have chiefly 

 to do with colour-sensation, and the rods with the discrimi- 

 nation of form. The sensitising substance, the so-called 

 " visual purple," found in the retina, changes under the 

 influence of light and the resulting chemical changes stimu- 

 late the nerve- endings, with sensation as the result. In the 

 retina of birds there are red and yellow oil globules, of 



