CHAP, ii.] SIGHT. 531 



from red blindness is impossible, the only two possible colour 

 defects being red-green and blue-yellow blindness. And the 

 existence of distinct green blindness has been held to contradict 

 that theory. On the other hand the Hering theory admits the 

 possibility of total colour blindness, i. e. the inability to see anything 

 but white and black ; and this, on the Young-Helmholtz theory, is 

 impossible, since for vision to exist at all, one of the three primary 

 sensations must be present ; a man to see at all must see things 

 in various shades of either red, or of green, or of violet, though he 

 may confound this single-coloured vision with the normal vision of 

 white of different intensities. But indeed a full examination of 

 colour blindness rather increases than diminishes the difficulties of 

 deciding between the two rival theories. 



Influence of the pigment of the yellow spot. In the macula 

 lutea, which part of the retina we use chiefly for vision, images 

 falling on other parts of the retina being said to give rise to 

 'indirect vision,' the yellow pigment absorbs some of the greenish- 

 blue rays. Hence the sensation which .we receive from objects 

 which we are in the habit of calling white is that which, if this 

 pigment were absent, we should receive from objects more or less 

 yellow. We may use this feature of the yellow spot for the purpose 

 of making the spot, so to speak, visible to ourselves, by an experi- 

 ment suggested by Maxwell. A solution of chrome alum, which 

 only transmits red and greenish-blue rays, is held up between the 

 eye and a white cloud. The greenish-blue rays are absorbed by the 

 yellow spot, and here the light gives rise to a sensation of red ; 

 whereas in the rest of the field of vision, the sensation is that 

 ordinarily produced by the purplish solution. The yellow spot is 

 consequently marked out as a rosy patch. This very soon however 

 dies away. 



In speaking of sensation as a function of the stimulus, p. 520, 

 we referred to white light only; but the different colours are 

 unequal in the relations borne by the intensity of the stimulus, 

 to the amount of sensation produced. Thus the more refrangible 

 blue rays produce a sensation more readily than the yellow or red 

 rays. Hence in dim lights, as those of evening and moonlight, 

 the blues preponderate, and the reds and yellows are less obvious. 

 So also when a landscape is viewed through a 3 r ellow glass, 

 the yellow hue suggests to the mind bright sunlight and summer 

 weather, although the actual illumination which reaches the eye is 

 diminished by the glass. Conversejy when the same landscape is 

 viewed through a blue glass the idea of moonlight or winter 

 is suggested. 



The theory of three primary colour sensations may be used 

 to explain why any coloured light, if made sufficiently intense, 

 appears white. Thus a violet light of moderate intensity appears 

 violet because it excites the primary sensation of violet much more 



342 



