, 



RETINAL EXCITATION 385 



colour-blindness there is no deficiency in the cones of the substance 

 on which the luminosity of the light depends (receptors), but, as 

 in deuteranopia, there is a reduction in the number of the sensa- 

 tions aroused by the stimulators. 



On Schenck's theory blue-yellow blindness can be explained 

 as due to anomalous development, in which the second phase in 

 the cleavage of the primitive substance reaches a certain stage 

 without the first having taken place. 



(cT) Total colour-blindness (achromatopsia), which is normal to 

 the most peripheral part of the retina, may under various abnormal 

 conditions, congenital or acquired, extend over the whole retina. 



One form of achromatopsia depends on functional absence of the 

 cones, while the rod-functions persist. In this the distribution of 

 brightness in the spectrum coincides with that observed in scotopic 

 vision and where there is a central scotoma. Several clinical cases 

 have been well investigated, and afford complete confirmation of 

 v. Kries' duplicity theory of the functions of the rods and cones. 



A second variety of achromatopsia differs from the preceding 

 only in there being no central scotoma. These cases are well 

 explained on Schenck's theory by assuming that the evolution of 

 the cones became arrested at the first stage when only the primi- 

 tive visual substance was present, before it had undergone cleavage ; 

 consequently the sensation of white light is alone perceptible. 



A third variety of achromatopsia is characterised by that dis- 

 tribution of brightness in the spectrum which is observed in the 

 outer zone of the retina in deuteranopes, due, on Schenck's theory, 

 to the absence of receptors for rays of long wave-length. In two 

 clinical cases examined by Konig the peripheral colour-blindness 

 of protanopes extended to the centre of the retina. In one case 

 .there was total colour-blindness ; in the other a trace of yellow 

 and blue sensation persisted. 



In conclusion, a fourth kind of achromatopsia is that seen at 

 the retinal periphery in deuteranopes, where the distribution of 

 luminosity in the spectrum is the same as that of normal people, 

 which on Schenck's theory is due to the fact that all the 

 receptors are associated with all the stimulators. There are cases 

 of congenital and pathological achromatopsia in which the peri- 

 pheral colour-blindness of deuteranopes and normals extends over 

 the whole retina. Congenital cases have been described by Beckef 

 and by Piper; clinical cases by Scholer and Uthoff, Siemerling 

 and Konig, and Pergens. 



Schenck's theory is an ingenious and very complicated hypo- 

 thesis to account for all the functions of the retina, particularly 

 the different varieties of partial or total colour-blindness which 

 could not be interpreted by any of the previous theories. 



It is undoubtedly a marked advance on those of his predecessors, 

 of which it is to a large extent the elaboration. 



VOL. iv 2 c 



