1910.] 



Light Perception to Colour Perception. 



461 



persons whose spectrum is shortened at the violet end. Although such 

 cases will have an ap2')reciation of colour differences as good as that of 

 the average individual, their colour sense will not be identical with the 

 latter. 



This will be evident if we consider the inliueuce of a shortened spectrum 

 upon colour vision. The first evident fact is that bodies reflecting only light, 

 the rays of whicli occupy the missing portion of the spectrum, appear 

 black. Nearly all colours are compound, that is to say, the coloured body 

 reflects other rays than those of the colour seen. Thus a blue-green glass 

 may transmit the green, blue, and violet rays of the spectrum. Let us 

 suppose that we have a substance reflecting the green, blue, and three- 

 quarters of the violet, the colour of the body to a normal person being green. 

 Then if we had another substance which reflected the whole of the violet, 

 it would appear l)lue. But ♦with a person who could not perceive the 

 terminal fourth of the violet the colour woidd look exactly the same as the 

 green one, and as he could not distinguish between the two, he would be in 

 continual difficulty with blues and greens. All coloured objects reflecting 

 rays occupying the missing portion appear darker than they do to the 

 normal sighted, and are always matched with darker colours belonging to a 

 point more internal. Thus a dichromic with a shortened red end of the 

 spectrum matches a red with a darker green. 



It will be noticed that a shortened spectrum, especially if one end only be 

 affected, may interfere very little with the general appreciation of shade. 

 If, for instance, we take a case in which the red end of the spectrum is 

 shortened, so that only three-quarters of the red of the normal sighted is 

 seen, then all bodies which equally reflect or transmit these rays can be 

 correctly compared, Ijecause a similar portion of light has been removed from 

 each. It is only when one colour reflects or transmits the rays occupying 

 the shortened portion and the other does not that there is any definite 

 interference with the appreciation of shade. Again, if neither colour reflects 

 or transmits rays occupying the shortened portion of the spectrum, there will 

 obviously be no interference with the appreciation of shade. 



A very common mistake due to shortening of the red end of the spectrimi 

 is the confusion of pink and blue. If a person with consideral)le shortening 

 of the red end of the spectrum is shown a pink which is paade uj) of a 

 mixture of red and violet, the red consisting of rays occupying the missing 

 portion of the spectrum, only the violet is visible to him, and so the pink 

 appears a violet without a trace of red. This pink is therefore matched with 

 a violet or blue very much darker than itself. 



Mistakes which are due to shortening of the spectrum may be remedied 



