CHAP. in.J SIGHT. 97 



white, whatever may be the colour sensation produced at the 

 same time ; a fully saturated colour, one wholly unmixed with 

 white, according to this view does not exist. This assumption 

 enables us to explain much more readily than does the Young- 

 Helmholtz theory the occurrence under certain circumstances 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 corresponding 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 there- 

 fore leave these two conditions of equilibrium on one side. 

 But we are constantly experiencing the condition of equili- 

 brium 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 sub- 

 stance 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 sensation 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 experience 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 corresponding 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 corresponding 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 sensation of the "intrinsic light of the retina." 



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