THE SUBJECTIVE PHENOMENA OF VISION 571 



to see with the dazzled part of the retina. If a field is looked 

 at, a black spot appears to lie in front of it ; if on the other 

 hand the field subsequently looked at is dark, this same area of the 

 retina appears to be filled by a bright haze. If the dazzling light be 

 restricted to one colour, there is an inability to see the same colour, 

 if of lower intensity, immediately afterwards. The power to see other 

 colours is apparently quite unaffected ; in fact. Burch stated that the 

 complementary colour actually appears to be more vivid than usual. 

 These changes are similar to those caused by after images. The negative 

 after image causes diminished appreciation of colours similar to itself, while 

 the positive shows itself as "a bright image similar in colour to the original 

 stimulus when a dark field is looked at. 



SUCCESSIVE CONTRAST. Visual impressions are affected by the 

 previous history of the retina ; thus after the eyes have been directed 

 towards a red surface, a grey surface appears to be tinted green, a green 

 surface seems a more vivid colour than normal, while a red colour is relatively 

 dull. In other words, after stimulation by one kind of source, another of a 

 similar nature is inhibited, while that of a different nature is either unaffected 

 or may be even increased. This effect is called successive contrast. Experi- 

 ment shows that the change of the second stimulus is such that it favours 

 the colour complementary to the first stimulus. These effects are 

 similar to those already described under adaptation and fatigue, and the 

 causation of the phenomenon is the same as'that given above, being due to 

 the presence of an after image. 



SIMULTANEOUS CONTRAST. If the retina is stimulated by two 

 separate impressions, any differences between the impressions will be 

 found to be accentuated. Thus if a small grey surface be placed on 

 a white ground, the grey will become darker and to a less extent 

 the white ground will become lighter. If now the same grey surface be 

 placed on a dark ground, it will be found to become lighter and the field 

 darker. The nearer the surfaces are together the greater are the effects 

 of contrast, the edges shewing the effects of contrast most. In 

 the case of colours similar changes take place ; thus two similar 

 colours of different intensity placed together appear to be more 

 different; two colours of different saturation change, the one to 

 greater, the other to less saturation ; while two colours of different 

 wavelength appear under the influence of contrast to suffer a varia- 

 tion towards the complementary colour. A similar change is observed if 

 contrast is occurring between a colour and a grey surface of approximately 

 the same intensity, for we find that the grey is very obviously tinted with 

 a colour that is very nearly the complementary of the colour in question. 

 It has been stated that the light which reaches the eye other than through 

 the pupil (e.g. the sclera) and which is coloured an orange-pink in consequence 

 of its partial absorption by the blood pigment in the capillaries, is re- 

 sponsible for the contrast colours not being strictly complementary to 

 those which produce them. It is also found that separation of the 



