904 THEORIES OF COLOUR VISION. [BOOK in. 



Young-Helmholtz theory the occurrence under certain circum- 

 stances of white sensations replacing or accompanying, that is 

 to say diminishing the saturation of, colour sensations. On the 

 other hand it introduces what appears to many minds a grave 

 difficulty in reference to black. The theory supposes that the 

 sensation of black is the result of the predominance of anabolic 

 changes in the white-black substance. But what name are we 

 to give to the sensation when the white-black substance is in a 

 condition of equilibrium? We cannot investigate the corre- 

 sponding conditions of equilibrium in the red-green, or in the 

 yellow-blue substance, because we can never study these by 

 themselves. When either of them occurs, as when rays limited 

 to certain wave-lengths are falling on the retina, we are by 

 hypothesis at the same time subject to changes in the white- 

 black substance ; we may therefore leave these two conditions 

 of equilibrium on one side. But we are constantly experienc- 

 ing the condition of equilibrium of the white-black substance, 

 unaccompanied by any stimulation of either the red-green or 

 yellow-blue substance ; we do so when the influence of light 

 has for some time been wholly removed from the eye, or again 

 taking the view, which is the more probable one, that the 

 changes of which we are speaking are cerebral changes, when 

 the retina by disease or injury has become insensible to light. 

 Under such circumstances we must suppose that the previous 

 katabolic excitement of the white-black substance has died 

 away, and that the substance is in equilibrium. Now when we 

 examine our sensation under these circumstances, we find that 

 though it is one of darkness it is one which differs from a sen- 

 sation of intense blackness. So distinct is the difference that 

 the sensation in question has been spoken of under the phrase 

 "the intrinsic light of the retina." And that we may experi- 

 ence sensations of black different from this sensation due to the 

 retina being at rest may be shewn in several ways. When we 

 close and shade the eyes after they have been exposed to a very 

 bright sunlight, we first experience a sensation of blackness, but 

 this soon gives way to the sensation of mere darkness corre- 

 sponding to the "intrinsic light of the retina." Again if we 

 stare for some time at a white disc on a black field and then 

 close the eyes, what we shall speak of presently as a negative 

 after image is developed ; the part of the field of vision corre- 

 sponding to the white disc appears as a black disc, which by its 

 blackness stands out in fairly strong contrast to the rest of the 

 field of vision, which corresponding to the area of the retina 

 previously free from the stimulus of light, now yields the sen- 

 sation of the "intrinsic light of the retina." And other 

 examples of a similar kind might be given. Admitting then 

 that the " intrinsic light of the retina " corresponds to a condi- 

 tion of equilibrium of the white black substance, we may speak 



