208 COLOUR VISION 



duplicity theory. The " night blindness " of the fovea, with its almost 

 if not quite complete absence of dark adaptation, the absence of 

 Purkinje's phenomenon on direct fixation with sufficiently small areas 

 of stimulation, the almost if not quite complete absence of a photo- 

 chromatic interval at the fovea, and so on have already been sufficiently 

 discussed, but should be referred to again in the light of the theory. 



The difference between the peripheral and the achromatic scotopic 

 luminosity curves, the former having the same character as the foveal 

 luminosity curve, shows, as has already been mentioned, that the 

 duplicity theory cannot be held to account for the peripheral total 

 colour blindness of the photopic eye. 



Temporal effects, such as those of recurrent vision, are so compli- 

 cated that the support they give to the theory is equivocal. MacDougall, 

 as has been seen, brought forward evidence to show that the " ghost " 

 is not necessarily, as was thought, a purely scotopic image, though it 

 is so in its typical form. The change in the fusion frequency in passing 

 from lights of low to lights of high intensity, as shown particularly in 

 T. C. Porter's experiments {v. p. 95), and confirmed in those of Ives {v. 

 p. 96), Dow, Schaternikof!^ and v. Kries^, demonstrate the change over 

 from the scotopic to the photopic mechanism. Particularly noteworthy 

 is the sudden change in value of the constant in Porter's logarithmic 

 equation, which is exactly parallel to the change in the constant for 

 Konig's logarithmic equation for visual acuity^ and is attributed to the 

 same cause. The agreement in the intensity, about 0"25 metre-candle, 

 at which the sudden change in the curve takes place, is striking. There 

 is also some evidence to show that the latent period of excitation of the 

 scotopic apparatus is appreciably longer than that of the photopic 

 apparatus. 



The experiments of Piper, Loeser, and Henius and Fujita {v. p. 123) 

 show that in the areal effects of the periphery as compared with the 

 fovea at low intensities summation of stimuli plays a much greater 

 role in the scotopic than in the photopic apparatus. 



Certain cases of abnormal vision afford valuable evidence in support 

 of the duplicity theory. Of these, total colour blindness or mono- 

 chromatic vision may be explained as due to functional abeyance of 

 the cones, and night blindness to functional abeyance of the rods. 



1 Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxix. 242, 1902, 



2 Ibid. XXXII. 113, 1903. 



3 Koiiig, p. 378 ; see Parsons, Roy. Lond. Ophth. Hosp. Rep. xix. 2, 283, 1914 ; 

 Cf. Bloom and Garten, Arch. f. d. ges. Physiol. Lxxn. 372, 1898. 



