t)26 A TEXTBOOK OF PHYSIOLOGY 



The black-white substance is stimulated also by the sensations- 

 of luminosity. If, therefore, red and green sensations of equal in- 

 tensity are thrown on the retina, they negative one another, and the 

 result is a sensation of luminosity. 



Against this view is the experimental fact that stimulation of the 

 retina by the different colour pairs does not produce electrical variations- 

 in opposite directions, as they should do if the substance were affected 

 in one case in an anabolic and in the other case in a katabolic direc- 

 tion. As the result of the exposure of the retina to coloured lights, 

 the current of action is in the same direction, but with different latent 

 periods (Fig. 349). 



Many experiments have been performed to support one or other 

 of these views. Considerable importance has been attached to the 

 evidence afforded by the condition of " colour-blindness." It would 

 appear that the varying conditions met with in colour-blindness are 

 not satisfactorily explained by either hypothesis. 



Colour-Blindness. Certain people are said to be colour- blind. 

 They are unable to distinguish certain colours or shades which are 

 easily appreciated by the normal eye. The defect is generally due 

 to one of two conditions, or a combination of them: (1) The person 

 is unable to perceive all the six colours normally seen in the spectrum ; 

 (2) the person is unable to appreciate the full extent of the spectrum, 

 especially of the red end. 



In the first group, all degrees of colour-blindness may be found. 

 Instead of perceiving six colours, an individual may only be able to 

 see two colours; he is therefore termed a dichromic. If he see three 

 colours, he is a trichromic; if four, a tetrachromic; if five, a penta- 

 chromic as compared with the normal person, who sees six colours, 

 and is hexachromic. A certain number of people perceive seven 

 colours, and are heptachromic. 



The commonest colours which are confused are red and green, 

 two forms of colour-blindness being distinguished the " red-blind "" 

 and the " green-blind." 



The " red-blind " is a dichromic, with a shortening of the red end 

 of the spectrum. He cannot see the extreme red rays. From the 

 nearer red rays, orange, and yellow, the red element is lacking, and it 

 appears as some shade of green. Green, violet, and blue, are normal. 



The " green-blind " is also a dichromic. The middle of the spec- 

 trum appears to him grey, with a patch of strong red gradually fading 

 into the grey on one side, and the violet merging into the grey on the 

 other. Such a person can see red well, and sees also a variety of shades 

 of rose. Greys and greens are confused, both being mistaken for red. 

 The case of a small boy is quoted who wanted to paint a picture with 

 the dust froin his boots, because it was such a pretty " rose " colour. 



Such persons get greatly muddled as the result of correction. 

 Thus, the boy referred to above, having been told that a green was 

 green, judged a donkey also to be green in colour. So confusion arises 

 to the red-blind. Having been told that yellow, which appears to 

 him green, is yellow, he confuses it with orange and red, which also* 



