vii KETINAL EXCITATION 365 



Charpentier and Tschermak contended against the theory of 

 v. Kries that in the fovea, unlike other parts of the retina in 

 which rods are present, the absolute threshold value of the 

 stimuli capable of arousing a luminous sensation coincides with 

 the chromatic threshold, so that on starting from subliminal 

 stimuli and gradually increasing their intensity, colour sensations 

 with no interval of colourless sensations are at once perceived. 

 But these observations do not overthrow v. Kries' theory, as he 

 states that the cones, on stimulation by monochromatic rays of 

 low intensity, give rise to colourless senations. 



Hermann's objection to v. Kries' theory is still less conclusive. 

 He contends that by it there must, in cases of achromatopsia or 

 congenital colour-blindness, be a gap or scotoma in the visual 

 field corresponding to the fovea but this is not found to be 

 the case. It is conceivable that in achromatopsia, owing to arrest 

 of functional development, the cones may not have acquired the 

 capacity of arousing colour sensations, but only have the power, 

 common to the rods, of evoking simple colourless sensations. 

 There are, however, as we shall see, cases of achromatopsia with 

 central scotoma, which fully bear out v. Kries' theory. 



On the other hand, in typical cases of colour-blindness, when 

 light - adapted, the distribution of brightness in the spectrum 

 corresponds approximately to that found in the normal dark- 

 adapted eye, and is almost independent of the intensity of the 

 stimulus. Moreover, eyes that are colour-blind are dazzled in 

 full light, and their visual acuity is less than the normal average. 

 All these facts agree perfectly with the hypothesis that achroma- 

 topsia is due to defective development in the functional capacity 

 of the cones. But we shall consider this in detail, in reference 

 to particular cases. 



IX. Before putting forward any theory of colour-vision, it is 

 necessary to discuss the phenomena which result from the mixture 

 of colours. 



Experience shows that, as a rule, mixtures of objectively 

 different lights produce different effects upon the eye, and appear 

 to us as different colours; often, however, subjectively equal 

 lights or colours may be composed from mixtures of objectively 

 different lights. The so-called laws of colour-mixture, which are 

 of great physiological importance, have been obtained by methodica-1 

 investigation of the conditions under which the different colours 

 appear to us similar or different. 



Simple or. homogeneous lights or colours are those which result 

 from ether vibrations of a known wave-length. We have seen 

 that in the solar spectrum there is a series of imperceptible 

 transitions from red to violet. Objectively, therefore, we should 

 find an infinite range of simple hues, corresponding to the 

 different wave-lengths or vibration -periods. Subjectively, how- 



