CH. LVI.] AFTER-IMAGES 801 



After-images. These are the after-effects of retinal excitation, and are divided 

 into positive and negative. Positive after-images resemble the original image in dis- 

 tribution of brightness and colour. In negative after-images bright parts appear 

 dark, dark parts bright, and coloured parts in the complementary or contrast colours. 



If a bright white object is looked at, and the eyelids are then closed, a positive 

 after-image is seen which fades gradually, but as it fades it passes through blue, violet 

 or red, to orange ; according to the Young-Helmholtz theory, this is explained on the 

 hypothesis that the excitation does not decline with equal rapidity in the three colour 

 terminals. Further details of these* after-images are given on p. 803. A positive 

 after-image is readily obtained by momentarily looking at a bright object, e.g. a 

 window, after waking from sleep. Negative after-images develop later than positive 

 images, and may be seen either by closing the eyes or by turning them to a uniform 

 grey surface after viewing an object steadily. 



If the object looked at is coloured, the negative after-image seen upon such a 

 background is in its complementary colour ; this is explained by the Young-Helmholtz 

 theory, by the supposition that the colour perceiving element for the colour looked at 

 is the most fatigued, and the terminals for its complementary colour least fatigued. 

 On the Hering theory, one colour produces anabolic or katabolic effects as the case 

 may be; on withdrawing the eye from stimulation by that particular colour, the 

 opposite phase of metabolism takes place and produces the complementary colour. 

 One has an analogy to this in the case of the heart ; when that organ has been thrown 

 into an anabolic state by the stimulation of the vagus, it will beat better when the 

 stimulation stops owing- to increase of katabolic processes. 



Negative after-images are frequently spoken of as phenomena of successive con- 

 trast. Somewhat more complex than these are the phenomena of simultaneous con- 

 trast. A white object looks whiter when viewed against a dark background than when 

 seen against a white background ; the colour of an object is intensified by viewing it 

 against a background of its complementary hue. Another familiar experiment is ihe 

 following ; A piece of grey paper is placed on a green sheet, and the whole covered 

 with tissue paper ; the grey patch then appears to be reddish, that is, of the colour 

 complementary to green. Helmholtz regarded such phenomena as the result of false 

 judgment, and not of changes in excitability of the different parts of the retina. * It 

 certainly appears easier to explain contrast by the Hering theory ; excitation by one 

 colour induces an opposite metabolic process in neighbouring areas of the retina ; if 

 two stimuli of opposite character are presented simultaneously side by side, the con- 

 trast effect will be most marked. In the experiment with tissue paper, the greater 

 part of the retina is being excited by green, and the part of the retina stimulated by 

 the feeble white light from the grey paper will undergo the opposite change and pro- 

 duce a sensation of red. 



By means of the stereoscope, binocular combinations of colour can be obtained. 

 Thus, if one eye is exposed to a red disc, and the corresponding portion of the other 

 eye to a yellow one, the mind usually perceives one disc of an orange tint ; but 

 frequently, especially if there be differences of brightness or of form in the two 

 objects, we notice that "rivalry of the fields of vision" occurs, first one then the 

 other disc rising into consciousness. A stereoscopic combination of black and white 

 produces the appearance of metallic lustre ; this is very beautifully shown with 

 figures of crystals, one black on a white ground, the other white on a black ground. 

 Probably the combination of black and white is interpreted as indicating a polished 

 surface, because a polished surface reflects rays irregularly so that the two eyes re- 

 ceive stimuli of unequal intensity. 



Changes in the Retina during Activity. 



The method by which a ray of light is able to stimulate the 

 endings of the optic nerve in the retina in such a manner that a 



* By "retina" here and elsewhere we mean " cerebro-retinal apparatus.'' We 

 have no knowledge of the precise share of retina and brain in the development of 

 visual sensations and after-sensations. 



