500 ADAPTATIONS TO PHOTIC QUALITY 



work, though based upon colored papers rather than filtered lights 

 (which the Canadians used) , yielded rather more useful information. 



The budgerigar shows neither the blue-blindness nor the extra sensi- 

 tivity to red exhibited by the domestic fowl and other birds. Supposedly, 

 this is due to the fact that this bird lacks the deep red oil-droplets present 

 in both hen and pigeon. According to Plath, the parakeet has only 

 orange, yellow, and pallid greenish droplets. The species discriminates 

 blues and violets from grays about as readily as other colors. Grays are 

 distinguished from one another with difficulty (and are perhaps never 

 seen photopically, by any bird, untinged by oil-droplet colors). The 

 curve of hue-discrimination has two maxima, somewhere in the yellow- 

 green and in the short-wave regions — they could not be precisely located 

 with Plath's colored-paper technique. Violet was as often confused with 

 red as with blue, indicating a closed color circle. 



Though many investigators have demonstrated a Purkinje phenom- 

 enon in diurnal birds by means of pupilloscopic, electroretinographic, 

 and training techniques, not much has been done by way of a compari- 

 son of the photopic vision of a single species with its own scotopic vision. 

 Rather, the photopic vision of diurnal birds has been contrasted with the 

 photopic vision of nocturnal birds, and a little has been done with the 

 scotopic vision of the latter. 



Piper, in 1905, was the first to make such comparisons. He recorded 

 the retinal action currents under monochromatic lights, and found that 

 the eyes of diurnal birds, such as the hen and buzzard, all gave maximal 

 responses to A,600m[X, both when light- adapted and dark- adapted. Owls, 

 both scotopically and photopically, proved most sensitive to A,535m[l. 

 A Purkinje phenomenon for either type of bird was thus denied, though 

 one might speak here of an 'interspecific Purkinje phenomenon', bearing 

 out the Duplicity Theory just as well; for the diurnal birds have few 

 rods and the owls, few cones. 



But no bird is known to have an absolutely pure-cone or pure-rod 

 retina, though some are suspected of having no rods and the most noc- 

 turnal of all birds {Apteryx? Steatornis?) may, when studied histologi- 

 cally, prove to have no cones. All duplex birds should show a Purkinje 

 phenomenon, and Piper's results have consequently been questioned 

 many times. In 1907, Abelsdorff first applied to birds the then recent 

 discovery of M. Sachs : that the responses of the pupil to lights indicate 

 directly the relative brightnesses of the lights to the animal. He found 

 the pigeon's pupil to be less responsive to green and blue than the human 



