288 COLOUR VISION 



protanopes, whereas beyond 580 /x/x violetwards the stimulus values are 

 relatively higher than for deuteranopes and normals. 



Hence protanopia is to be regarded as an arrested development in 

 which a receptor is absent, and that receptor must be the one which is 

 set in action by lights of long wave-length. Hence the receptor or 

 resonator of the red substances is absent, whilst the stimulator is 

 present. 



Tritanopia offers difficulties to the theory, but is explained as due to 

 the second developmental phase having reached a certain stage without 

 the first having taken place. 



Monochromatic Vision also offers special difficulties and is probably 

 complex. Schenck distinguishes three types : (1) the peripheral total 

 colour blindness of normals and deuteranopes ; (2) the peripheral total 

 colour blindness of protanopes ; (3) scotopic total colour blindness. 

 He concludes that (1) the white visual substance of the ro4s develops 

 in such a manner as to become relatively more sensitive to rays of 

 medium wave-length, thus producing the white substance of the 

 protanopic cones ; (2) from the protanopic cone white substance the 

 normal peripheral cone white substance is developed by further differen- 

 tiation, so that it becomes more sensitive to rays of long wave-length. 



Complete monochromatic vision may be due to (1) pure rod- vision ; 

 (2) arrest of cone development at the rod stage (total colour blindness 

 without central scotoma) ; (3) arrest of cone development at the 

 protanopic peripheral cone stage (pathological cases) ; (4) arrest of cone 

 development at the normal peripheral cone stage (the cases of Becker^, 

 Piper^, Scholer and Uhthoff^, Siemerling and K5nig^, and Pergens^). 



Ationialous Trichromatic Systems. Partial deuteranopia is caused by 

 the receptor for light of medium wave-length possessing an absolutely 

 diminished sensitiveness, with relatively diminished sensitiveness for 

 about 560 y^t/x, and relatively increased sensitiveness for about 580 fifj.. 

 Partial protanopia is caused by absence of the receptor for light of 

 long wave-length and modification of the receptor for light of medium 

 wave-length. 



It will be noticed that this theory is essentially the Young-Helmholtz 

 theory modified and elaborated so as to overcome certain difficulties. 



1 Arch./. Ophlh xxv. 2, 205, 1879. 



^ Ztsch. f. Psychol, u. Physiol, d. Sinnesorg. xxxvni. 155, 1905. 



^ Beitrdge z. Path. d. Sehnerven, Berlin, 1884. 



* Arch, f Psychint. und NervenJcr. xxi. 284, 1889 ; in Konig, p. 206. 



* Klin. Monaishl. f. Augenhlk. XL. 42, 46, 1902. 



