1896-97-] SPATIAL THRESHOLDS OF COLOURS. 239 



in his article " Beitrage fiir Kenntnisse der Farbenblindheit," in Vol. 

 VIII, of Philos. Studien, p. 199. It was the case of Prof. A., numbered 

 case V, n the forementioned article— a man whose left eye was perfectly 

 normal in its colour sensibility, but whose right eye was colour blind, 

 not by accident or disease, but congenitally. The right eye for this 

 subject was a dichromate, i.e. possessed of only two main colours in its 

 colour system, namely, red and blue, and these behaved throughout for 

 that eye as complementaries, as shown by the after image tests. It is 

 rather a peculiar fact that in the normal eye, as our experiments have 

 shown, for small angular sizes of coloured surfaces the same complemen- 

 tary relation of blue and red appears to obtain, as was evinced in the 

 abnormal eye of Prof A. Whatever the reason may be accounting for 

 the fact, the coincidence at any rate is somewhat striking, the more so 

 because the peculiar tendencies of the red and blue were so uniquivocally 

 established so far as our experiments at least were concerned. 



It may be noticed in closing this paper that this peculiar behaviour 

 of blue and red in regard to their characteristic space thresholds when 

 respectively under the other's contrast influence seems to form a serious 

 difficulty to Hering's colour theory which explains all colour phenom- 

 ena by means of a threefold antagonism of fundamental colour 

 processes in which black counterpoises white, red counterpoises green, 

 and yellow, blue. This theory would claim that the red process which 

 signifies the destruction of nervous elements of a certain kind should 

 originate the green process, both in the recuperation of those same 

 elements and in the neighbouring ones also, which have been disturbed. 

 In other words specifically applied to our apparatus a red ground 

 when looked at for the emergence of another colour within its limits as 

 would occur in the central opening of a red coloured diaphragm to 

 permit the entry of other coloured light, should tend to induce upon 

 that element in the red surface a green color ; because, according to 

 Hering's theory the great activity of those retinal elements stimulated 

 by the red ground, should tend to induce a neighbouring antagonistic 

 activity in the retinal elements which are free from the stimulation of 

 the red ground, namely, those upon which the emerging light of the 

 diaphragm aperture falls. But our experiments show a quite contrary 

 state of things. Instead of the ;r^ ground inducing \\\& green, and the 

 blue ground the orange or yellow which the Hering theory demands, we 

 actually find as before indicated the red ground inducing blue, and the 

 blue inducing red quite in defiance of what that theory pronounces 

 should be. Perhaps the adherents of Hering's theory might be inclined 

 to try to explain the phenomenon by irradiation or what Hering calls 



