24 OLD FLIES IN NEW DRESSES 



of the colour-blind may be divided into two 

 sub-classes those who are red-blind and 

 those who are green-blind. 



Those who are red-blind do not see the 

 red end of the spectrum, and the blue-green 

 appears grey, though they have distinct 

 colour vision of the parts of the spectrum 

 on either side of the blue-green. In match- 

 ing red with a green, they put a bright red 

 with a dark green. 



On the other hand, those who are green- 

 blind see the red end of the spectrum, 

 while the green appears to them as grey. 

 In matching a red with a green they put a 

 dark red with a bright green. 



No absolutely undoubted cases of blue- 

 yellow blindness have been recorded, and 

 only one of absolute colour-blindness ; but 

 one case is not sufficient to go upon. 



According to the Young - Helmholtz 

 theory, a case in which only shades of 

 black and white were visible would be 

 impossible, as it would not be shades of 

 black and white which would be seen, but 

 shades of either red, green or blue. 

 According to Bering's theory, of course 

 absolute colour-blindness would be possible. 



