502 ADAPTATIONS TO PHOTIC QUALITY 



is in perfect keeping with the relative numbers of deeply colored oil-drop- 

 lets in the three types (see p. 197). Long-wave light had little stimulating 

 value for owls. Ten years later, however, Vanderplank made the surpris- 

 ing aimouncement that the tawny owl, Strix aluco, has a band of visi- 

 bility in the (human) infra-red, and thus sees its prey, in what for man 

 would be pitch darkness, by means of the prey animal's own body heat. 

 Vanderplank found that a strong beam of A,900m[i closed the owl's pupil 

 and seemed to dazzle and frighten the bird, though it had no effect on 

 the human eye. The owl could not find dead, cold prey or chunks of 

 meat in a darkroom — unless they were illuminated by an infra-red spot- 

 light. 



Hecht and Pirenne have lately published contradictory findings, 

 though to be sure not on a close relative of Vanderplank's species of owl. 

 Working with Asio wilsonianus, the Americans found the curve of pupil- 

 lomotor effectiveness to be identical with the human scotopic brightness 

 curve (Fig. 35, p. 102), indicating that the photochemical system of the 

 owl's rod is the same as that of our own, and contains nothing in addition 

 to rhodopsin which could give it responsiveness to 'black' light. Vander- 

 plank might have been more perfectly refuted if Hecht and Pirenne had 

 chosen to work on Strix varia instead of Asio; but recently Matthews 

 and Matthews, studying S. aluco, have claimed that the eye makes no 

 response to black-body radiations from 40 C to 400 C, and that the 

 transmission of long infra-red wavelengths through the ocular media 

 is nil. 



The spectral sensitivities of such birds, whose vision is certainly entirely 

 achromatic, are of little general interest. But considerable speculation has 

 been offered as to how the world of hues appears to diurnal birds. Natu- 

 rally, it depends upon the kind of bird — particularly, upon interspecific 

 differences in the oil-droplet mosaic. Where red droplets are numerous, 

 as in song-birds and fowls (and particularly in kingfishers), blues and 

 violets must be seen weakly and unsaturated. Hawks and woodpeckers 

 have few red droplets, parrots perhaps fewer still, or even none in some 

 species. The primary function of the droplets is not to produce hue-dis- 

 crimination (see p. 193) ; but they do necessarily influence the appear- 

 ance of colored objects profoundly. Tiny, uncontrollable eye movements 

 appose first one color of droplet, then another, then a colorless cone or a 

 rod, to a given point in the optical retinal image. Each point in space is 

 thus continuously 'scanned' by a succession of filters; and while at any 

 one instant these abolish as many contrasts as they enhance, in the next 



