224 Lewis, Some Considerations on Sight in Birds. [isfAprii 



appreciated by the stimulation of waves of varying lengths. In 

 man it varies from about 770 ^. to 396 jx., the latter being the 

 extreme of light registered at the violet end of the spectrum. 



It would appear, if we adopt the Young-Helmholtz theory, 

 that man has a trichromatic vision, and that all the shades 

 appreciated are due to the degree in which the three classes of 

 nerve fibres are stimulated. Yellow, for example, is caused by 

 an equal stimulation of the sets of fibres for the red and green 

 percipients. When red is seen, the fibres percipient of red are 

 strongly stimulated, the others only weakly. Colour-blindness is 

 an interesting side study in this respect, particularly when we 

 come to the colour vision of birds. In man dichromatic vision 

 appears most commonly with a l)lindness for red or green, the 

 violet blind being rare. In red or green l)lindness. the subject 

 confuses reds and greens, and in a mixture of colours including 

 these, colours other than red or green are the only ones appreciated. 



Now, it has been shown by feeding experiments that birds are 

 blind in the violet end of the spectrum. In other words, if we 

 accept the Young-Helmholtz theory they have a dichromatic 

 vision. Their colour vision would be restricted to red and green 

 and the mixtures of these colours. They would be blind to violet 

 and to the spectral violet in blue, indigo, and yellow. Such a 

 conclusion would be disastrous to our theory of selection in the 

 coloration of birds, where many blues and shades of blue are seen. 

 It would mean that the development of colour in the evolution 

 of the present-day bird was merely incidental, and apjiarently 

 without reason. The flaw in the reasoning probably lies in our 

 acceptance of the Young-Helmholtz theory instead of recognizing 

 the other colours as primary. Again, the conclusion obtained 

 from the feeding experiments may be faulty. The birds are fed 

 in spectral red light and in spectral green, where they pick up the 

 grains readily ; but, when taken to spectral violet, remain still, 

 fail to see the grains, and are to all intents and purposes in 

 darkness. 



A man colour-blind in red or in green, though not seeing these 

 colours as a normal person would see them, still sees the objects, 

 but is blind to the colour only. His vision extends right to the 

 red end of the spectrum, though not recognizing the red there, 

 so that the waves stimulate the eye, though not giving the colour 

 sense. It is probable that in birds the sight is keyed to a higher 

 pitch than in man, and that the retina is not stimulated by wave 

 lengths as short as that of the violet, while yet possessing the 

 whole of the range of colours as far as the violet. In man we 

 know that the eye is blind beyond the two limits of red and violet, 

 but we are able to ascertain the presence of ultra red and ultra 

 violet rays that the retina does not register. 



There is still a great field for investigation into the function of 

 sight. So far, the work done is mainly comparative, and must 

 be based on the lines found existent in the human subject, where 

 the subjective assistance is of great value. But of the conditions 



