210 COLOUR VISION 



Parinaud^ regarded night blindness as strongly supporting the 

 views which he propounded and which have since become incorporated 

 in the duplicity theory. Night blindness may be looked upon as the 

 obverse of total colour blindness. In the latter rod-vision is isolated, 

 in the former rod-vision is seriously disturbed or in abeyance. 



In night blindness dark adaptation may be almost abolished or 

 much slower than normal ; in high degrees it is both slowed, and 

 diminished quantitatively, i.e., the higher degrees of sensitiveness of 

 the retina are never reached and the highest possible are reached only 

 after prolonged exclusion of light from the eye^. In low degrees central 

 vision is normal in the light-adapted condition, and may be so in high 

 degrees of the disease, e.g. in retinitis pigmentosa. Colour vision is 

 normal with the exception of occasional diminution of sensibility for 

 blue lights. In retinitis pigmentosa the field of vision is diminished, 

 often almost down to the fixation point, and dark adaptation may be 

 completely absent. 



Purkinje's phenomenon is much less marked in the night-blind. 

 If a red and a green are chosen, which are equal in luminosity for the 

 normal and night-blind in light adaptation, and the room is then 

 darkened the green becomes much brighter than the red in the course 

 of a few minutes for the normal, but only after a long time, if at all, 

 for the night-blind. Quantitative experiments show that for lights of 

 short wave-length the increase in luminosity after half-an-hour's dark 

 adaptation is 10 to 100 times as great for the normal as for the night- 

 blind. With stimulation areas exceeding the foveal limits the sensi- 

 bility for mixed white lights increases much more rapidly for the normal 

 than for the night-blind on dark adaptation, but if both are tested with 

 red lights there is very little difference. This is further proof of the 

 diminution in the appreciation of Purkinje's phenomenon by the night- 

 blind. 



The temporal effects in the night-blind have not been investigated 

 as thoroughly as could be wished, and the results are not concordant. 



The condition is often, in fact generally, partial. It is not sur- 

 prising therefore that cases occur in which Purkinje's phenomenon 

 occurs and which show some degree of dark adaptation, colourless 

 interval for red pigments and so on. Hess^ has laid great stress upon 



^ Arch. gen. de mM. 1881 ; C. r. acad. des sci. 1881 ; La Vision, Paris, 1898. 



^ Heinrichsdorff, Arch. f. Ophth. lx. 405, 1905; Messmer, Ztsch.f. Sinnesphy^iol. xixi. 

 83, 1907; Lohmann, Arch. f. Ophth. lxv. 3, 1907; Stargardt, op. cit. Lxxni. 1, 77, 1909; 

 Behr, op. cit. Lxxv. 201, 1910 ; Wolfflin, op. cit. lxxvi. 464, 1910. 



3 Arch. J. Avgenhlk. lxii. 50, 1908 ; LXix. 205, 1911. 



