Data on Colour-Blindness. 107 



colours used in the tests, will give, at a glance, a perfect 

 idea of the phenomena of vision of the dichromic patient ; 

 and by the simple application of a pair of compasses, the 

 quantitative determinations may at once be found. 



It will now be easily seen that the difference between this dia- 

 gram and Maxwell's normal one exactly expresses the essential 

 difference between normal and dichromic vision. In the 

 former the most salient feature is the immense variety of 

 hues ; the normal-eyed person has not only six chief colours, 

 each one differing essentially from all the rest, but their com- 

 binations with each other afford an infinity of additional 

 varieties of hue. This variety is wanting in dichromasie, but its 

 place is supplied by an infinity of varieties of tint and shade. 



Now in both diagrams the area of the figure may be filled 

 with innumerable points, each of which will express a different 

 variety of Colour-impression ; but while in the normal case 

 these are varieties of hue, in the dichromic case they are 

 varieties of tone. 



The want of appreciation of this difference becomes con- 

 tinually evident in communications between normal and 

 colour-blind people, the two parties being frequently quite at 

 a loss to understand each other ; and it accounts for some of 

 the mistaken views held by normal-eyed persons, even those 

 who have studied colour, as to what our impressions really are. 

 The errors often made by them in describing our sensations 

 seem quite as ludicrous to me, as any attempts of mine to 

 identify the normal colours would be to them. 



The same consideration influences the discussions on the 

 the alleged danger with railway signals. The alarmists think 

 it quite sufficient to say that " the colour-blind are unable to 

 distinguish between red and green." But this is speaking 

 in an unknown tongue. The point to test is, " whether the 

 peculiar variety of colour-impression made on the dichromic 

 eye by the red-lamp rays is so different from that made by 

 the green-lamp rays, that the distinction between them is 

 easily perceived." This is a form of the problem not often 

 thought of. 



Fig. 1 is the diagram of my own vision. Fig. 2 is that of 

 Mr. Parry, which forms the greatest contrast to mine. It is 

 not necessary to give the intermediate ones ; but the im- 

 portant results are recorded in the following table, which will 

 be a useful addition to the simple elementary data. 



