VISUAL SENSATIONS 649 



a dichromic with shortening of the red end of the spectrum. The 

 ' green-blind ' person is a dichromic without shortening of the red 

 end. It is an instructive experience to make either a dichromic 

 or a trichromic mark out on a spectrum a monochromatic patch. 

 In a dichromic, such a patch at the red end will include red, orange, 

 yellow, and green. In a trichromic, red, orange, and yellow will 

 probably be included in the patch. This method is the most 

 accurate way of determining diminution of the power of colour dis- 

 crimination and shows with ease even the minor degrees of colour 

 blindness. 



CONTRAST PHENOMENA 



Simultaneous Contrast. If a grey disc be placed on a piece of 

 red paper, and the whole covered with tissue paper, the disc will 

 take on a greenish tinge. If the ground colour be green, the 

 disc will appear red ; if blue, the disc will appear yellow ; in 

 fine, whatever be the ground colour, the colour of the disc will be 

 complementary to it. These effects are spoken of as simultaneous 

 contrast. 



Successive Contrast. If, after gazing steadily for some time at a 

 red disc on a white surface, the eyes be turned towards a plain white 

 surface, a negative after-image of the disc is seen on the paper 

 coloured green, i.e. the complementary colour of the red disc. 

 Surrounding this the paper appears red. If we look at the sun 

 for some time, and then turn our eyes away, there is at first a 

 positive after-image, and we see a bright sun wherever we look. In 

 a short time this disappears and gives way to a black sun (a negative 

 after-image). Thus we may say that stimulation of any part of the 

 retina with any colour is followed by a colour-sensation referred to 

 the same part of the visual field and complementary to the first. 



It has been much discussed whether these phenomena are simply 

 effects of judgment, or whether they are produced by definite changes 

 taking place in the retina. Helmholtz explains them by the first 

 hypothesis, and looks upon them as cerebral processes. Hering, on 

 the other hand, has extended his theory so as to embrace these 

 phenomena, and ascribes them to definite changes in the retina, or 

 at any rate in the peripheral part of the visual mechanism. A corollary 

 to his theory that we mentioned above is that if dissimilation of a 

 visual substance be excited at any point of the retina, assimilation of 

 the same substance is set up in the parts of the retina immediately 

 adjoining that point. To this process the name of ' retinal induction ' 

 has been given. In this way the phenomena of simultaneous contrast 

 may be explained. Thus if a ray of red light falls on any spot, it 

 may be supposed to excite dissimilation of the red-green substance 



