COLOR VISION IN BIRDS 503 



instant the pattern changes kaleidoscopically and the net result is en- 

 hancement of every contrast sooner or later — and all within a tiny frac- 

 tion of a second — making for a net improvement in visibilities in general. 



The oil-droplets cannot, however, increase brightnesses. Though the 

 red and orange ones may be held accountable for £he partial blue-blind- 

 ness of so many birds, they cannot possibly be what makes the same birds 

 extra-sensitive to red light. Any such peculiarity is due to the photochem- 

 ical properties of the cones and to their high concentration in the retina. 

 We, too, would probably see reds more vividly in the retinal periphery, 

 if the latter were pure-cone like the fovea. The rods being blind to red 

 light, their interposition in large numbers between the cones is analogous 

 to sprinkling a piece of red paper with gray dots : at a little distance the 

 paper will appear homogeneous but unsaturated, its red chroma weak. 

 Rods lying between the cones of any duplex retina naturally unsaturate 

 all colors by intermingling a grayness-sensation with the colored one 

 from the cones; but in the case of red, they introduce darkness, for they 

 do not 'see' red even as grayness. 



Hess was fond of saying that the bird sees the world as we would see 

 it through a pair of orange spectacles. Such a description perhaps covers 

 the dimming of short-wave stimuli, but scarcely the brightening, for the 

 bird, of long-wave ones. Moreover, though the blend of the bird's red 

 and yellow oil-droplets may theoretically be orange, the bird does not 

 have the effect of an orange droplet in each and every cone. If our bird's 

 eye view of things were taken through spectacles composed checker- 

 board fashion of minute red, yellow, and colorless areas, each just large 

 enough to subtend one cone back in the retina, analogous to the screen 

 of a Finlay or Dufay color-photo, we should then be able to gather some 

 idea of how things look to birds. Such a screen would have no such 

 action as that of a homogeneous orange filter. 



The possibilities as to manipulation of the ratios of colors in the oil- 

 droplet mosaic are infinite ; and we may be sure that some of the extreme 

 ratios we can tally, as in hawks and parrots and kingfishers, and in the 

 red and yellow fields of the pigeon's retina, represent adaptations to 

 aspects of the various birds' ways of life, some of which are still quite 

 unsuspected. Some suggestions have already been given (pp. 195-8). 

 A promising viewpoint is that of Worth and Porsch, who, independently 

 of each other, have pointed out that red and 'fire' colors are extremely 

 common among the flowers which are visited by such birds as honey-birds, 

 humming-birds, etc., and which are dependent upon such birds for their 



