

VISUAL SENSATIONS 581 



the luminosity the spectrum appears trichromic, being composed of the 

 colours red, green, and violet. In colour-blind individuals this limitation 

 of the colours distinguished applies to all strengths of luminosity. Thus 

 the dichromic sees only red and violet, the trichromic sees red, green, and 

 violet. It is the dichromic cases to which the name of ' colour-blind ' has 

 been chiefly applied, the trichromic cases being often missed, or classified 

 simply as ' colour weak.' 



The name of ' anomalous trichomats ' has been applied to people who, while able 

 to discriminate most of the spectral colours, use abnormal proportions of the red 

 and green, when they mix these colours to form yellow. 



The ordinary ' red-blind ' person is generally 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 discrimination 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 



