VISUAL SENSATIONS. 775 



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 trans- 

 mits 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. 766), we re- 

 ferred to white light only ; but the different colors are unequal in the rela- 

 tions 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 yellow 

 glass, the yellow hue suggests to the mind bright sunlight and summer 

 weather, although the actual illumination which reaches the eye is dimin- 

 ished by the glass. Conversely, when the same landscape is viewed through 

 a blue glass the idea of moonlight or winter is suggested. 



The theory of three primary color sensations may be used to explain 

 why any colored light, if made sufficiently intense, appears white. Thus a 

 violet light of moderate intensity appears violet because it excites the pri- 

 mary sensation of violet much more than those of green and red. If the 

 stimulus be increased the maximum of violet stimulation will be reached, 

 while the stimulation of green will continue to be increased and even that 

 of red to a slight degree. The result will be that the light appears violet 

 mixed with green, that is blue. If the stimulus be still further increased 

 while the green and violet are both excited to the maximum, the red stimu- 

 lation may be increased until the result is violet, green, and red in the pro- 

 portions which make white light. And so with light of other colors. 



669. After-images. We have already seen that in vision the sensation 

 lasts much longer than the stimulus. Under certain circumstances, such as 

 particular conditions of the eye, an intense stimulus, etc., the sensation is so 

 prolonged that it is spoken of as an after-image. Thus, if the eye be directed 

 to the sun, the image of that body is present for a long while after ; and, if, 

 on early waking, the eye be directed to the window for an instant and then 

 closed, an image of the window with its bright panes and darker sashes, the 

 various parts being of the same color as the object, will remain for an appre- 

 ciable time. These images, which are simply continuations of the sensation, 

 are spoken of as positive after-images. They are best seen after a momentary 

 exposure of the eye to the stimulus. 



When, however, the eye has been for some time subject to a stimulus, the 

 sensation which follows the withdrawal of the stimulus is of a different kind ; 

 what is called a negative after-image, or negative image, is produced. If, after 

 looking steadfastly at a white patch on a black ground, the eye be turned to 

 a white ground, a gray patch is seen for some little time. A^black patch on 

 a white ground similarly gives rise on a gray ground to a negative image in 

 the form of a white patch. This may be explained as the result of exhaus- 

 tion. When the white patch has been looked at steadily for some time, that 



