436 ELEMENTARY PHYSIOLOGY less, ix 



green " blindness are not uncommon ; but another form 

 of colour-blindness in which blue and yellow cannot be 

 distinguished from each other is much more rare ; and 

 still rarer, though of undoubted occurrence, are the cases 

 of those who are n-holly colour-blind, i.e. to whom all 

 colours are mere shades of one tint. 



This peculiarity of colour-blindness is simply un- 

 fortunate for most people, but it may be dangerous if 

 unknowingly possessed by engine-drivers or sailors, parti- 

 cularly since red-green colour-blindness is most common 

 and red and green are exactly the two colours ordinarily 

 used for signals. It probably arises either from a defect 

 in the retina, which renders that organ unable to respond to 

 different kinds of luminous vibrations, and consequently 

 insensible to red, yellow, or other rays, as the case may 

 be ; or the fault may lie in the visual sensorium itself. 



For ordinary purposes colour perception may be most 

 easily and successfully tested by asking the person under 

 examination to make matches between skeins of coloured 

 wool. In this way it is found that a red-green colour-blind 

 person matches a red with a green skein. A more 

 satisfactory test than the matching of wools is furnished 

 by the use of coloured lights, and for a detailed in- 

 vestigation of the sensations of the colour-blind more exact 

 observations by help of the spectrum are necessary. 



The phenomena of colour-blindness can, to a certain 

 extent at least, be explained according to either of the 

 theories of colour-vision which have been given above. 

 Thus by the Ycjung-Helmholtz theory a red-green colour- 

 blind person lacks either the red-perceiving or the green- 

 perceiving structures normally present either in the 

 retina or the visual sensorium. According to the theory 

 of Hering they lack the red-green visual substance. 



