THE DUPLICITY THEORY 209 



Total colour blindness, as we have seen, is characterised by luminosity 

 values which correspond with those of the normal achromatic scotopic 

 eye. Comparison of the curves (Figs. 54, 55) shows their almost complete 

 identity. Whereas, however, the normal achromatic scotopic curve is 

 valid only for low intensities of objective lights the achromatopic curve 

 is valid for all intensities, though with the higher intensities photophobia 

 occurs. The process of dark adaptation pursues the normal course in 

 the totally colour-blind. Exposure to bright light causes prolonged 

 diminution of vision, and recovery follows the course which would be 

 expected if it were dependent upon restoration of the visual purple. 



All totally colour-blind people have marked diminution of central 

 visual acuity, and in many cases there is undoubtedly a central scotoma. 

 The curve of visual acuity with gradually increasing light-intensities 

 shows no sudden bend, as in normal-sighted people, but continues in 

 the same direction as for lower intensities until photophobia sets in 

 (Konig). The reduction in central vision tends to absence of binocular 

 fixation and divergence of the optic axes. We may conclude that the 

 foveal neuroepithelium, if it retains its function at all, approximates 

 in character to that of the normal rods. 



Uhthoff and v. Kries^ have shown that the flicker phenomenon in 

 the totally colour-blind follows the same laws as in normal achromatic 

 scotopia. 



Nagel^ has tested a monochromat with matches between orange- 

 yellow (600 nfi) and greenish-blue (490 fjufx) {vide supra). A monochromat 

 can make such a luminosity match at ordinary intensities of light, where- 

 as a trichromat must be dark-adapted and the intensities must be very 

 low. With the photopic eye the intensity {i.e., slit- width of the spectro- 

 photometer in millimetres) was 0-44. After one hour's dark adaptation 

 it was 0-8. Compared with the achromatic scotopic trichromatic, the 

 achromatopic photopic retina contains less visual purple : hence the 

 trichromatic achromatic scotopic value for greenish-blue light is lowered 

 by absorption relatively to the achromatopic. 



Night-blindness is found in several diseases of the eye, most commonly 

 in retinitis pigmentosa and allied conditions, least adulterated by other 

 pathological symptoms and signs in congenital night blindness. The 

 last-named is congenital and hereditary^, 



^ V. Kries, Ztsch. f. Psychol, u. Physiol, d Sinnesorg. xxxii. 113, 1903. 

 2 V, Helmholtz, 3rd. ed. n. p. 328. 



^ Nettleship, Trails. Ojthth. Soc. xxvii. 1907 ; Parsons, Pathology of the Eye, iv. 1400, 

 1908. 



P. c. v. 14 



