r , COLOUR VISION. 



the upper figure, marked P, is deduced from the observations of Professor Pole ; 

 while the lower one. marked K, is founded on observations by a very accurate 

 observer of the normal type. 



The only difference between the two diagrams is that in the upper one 

 the red curve is absent. The forms of the other two curves are nearly the 

 same for both observers. We have great reason therefore to conclude that tin- 

 colour sensations which Professor Pole sees are what we call green and blue. 

 This is the result of my calculations; but Professor Pole agrees with every 

 other colour-blind person whom I know in denying that green is one of his 

 sensations. The colour-blind are always making mistakes about green things and 

 confounding them with red. The colours they have no doubts about are cer- 

 tainly blue and yellow, and they persist in saying that yellow, and not green, 

 is the colour which they are able to see. 



To explain this discrepancy we must remember that colour-blind persons 

 learn the names of colours by the same method as ourselves. They are told 

 that the sky is blue, that grass is green, that gold is yellow, and that soldiers' 

 coats are red. They observe difference in the colours of these objects, and they 

 often suppose that they see the same colours as we do, only not so well. But 

 if we look at the diagram we shall see that the brightest example of their 

 second sensation in the spectrum is not in the green, but in the part which 

 we call yellow, and which we teach them to call yellow. The figure of the 

 spectrum below Professor Pole's curves is intended to represent to ordinary eyes 

 what a colour-blind person would see in the spectrum. I hardly dare to draw your 

 attention to it, for if you were to think that any painted picture would enable you 

 to see with other people's vision I should certainly have lectured in vain. 



On the Yellow Spot. 



Experiments on colour indicate very considerable differences between the 

 vision of different persons, all of whom are of the ordinary type. A colour, 

 for instance, which one person on comparing it with white will pronounce 

 pinkish, another person will pronounce greenish. This difference, however, does 

 not arise from any diversity in the nature of the colour sensations in different 

 persons. It is exactly of the same kind as would be observed if one of the 

 persons wore yellow spectacles. In fact, most of us have near the middle of 

 the retina a yellow spot through which the rays must pass before they reach 

 the sensitive organ : this spot appears yellow because it absorbs the rays near 



