348 Report of the Committee on Colour- Vision. 



In some cases seven colours are seen, and then the seventh 

 colour appears at the point where it should appear by theory. 

 In the first degree of colour-blindness only five colours, or points 

 of difference, are seen in the spectrum ; in the next degree four ; 

 in the next, three ; then two. Then a neutral band appears at 

 the blue-green junction, and this increases in size in different 

 cases until total colour-blindness is reached. Therefore, the 

 vision of the normal-sighted being hexachromic, the vision of 

 the colour-blind is pentachromic, tetrachromic, trichromic, or 

 dichromic. It will be noticed that the greatest difference is to 

 be found between the 3-unit and the 2-unit cases of colour- 

 blindness, the primary colours for each being quite different. 

 The two primary colours for the 2-unit are yellow and blue, 

 and they each represent half of the spectrum. In the case of 

 the 3-unit the three primary colours are red, green, and 

 violet. -Red combined with green forms yellow ; violet combined 

 with green forms blue ; so it is evident that these colours occupy 

 the positions which I theoretically allotted to them. 



The above refers to the number of approximate psycho- 

 physical colour units. An approximate psychophysical colour 

 unit is a portion of a physical series which contains physical 

 units that are not easily distinguished from each other, and 

 are so much alike as to be called by the same name. An abso- 

 lute psychophysical colour unit is a portion of a physical series 

 which contains physical units that cannot be distinguished 

 from each other even under the most favourable circumstances. 

 It will be seen that an approximate unit contains several abso- 

 lute units, but in each case the similarity between them is greater 

 than the dissimilarity; for instance, there are many hues of 

 red, but the character of redness enables them to be classed 

 together. 



The other chief cause of colour-blindness is shortening of one 

 or both ends of the spectrum. This is probably due to some 

 retinal defect, as neither light nor colour are perceived at the 

 shortened end. It is distinct and separate from diminished 

 psychophysical perception, which is due to defective size of the 

 colour-perceiving centre in the brain. 



Question — How would you establish the six or any other 

 number of colours with the person you were examining? — A 

 person who sees six will at once say so. I test with a 

 spectroscope provided with two shutters in the eye-piece, 

 showing- the examinee in the first instance red, orange, 3 T ellow, 

 and yellow- green, because these are of nearly equal luminosity. 

 I make him indicate the junctions of the colours ; any colours 

 can be cut off with the shutters, so that the person examined is 

 not able from one colour to guess the others. I use an ordinary 

 spectroscope (one prism), provided with shutters, as explained. 

 The actual procedure in using this test is as follows : — I ascertain 

 where the spectrum commences, where it terminates, what 

 colours are seen, and where the junctions of the colours are, the 



