KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 461 



little to the right, i.e., to wave-lengths about 35 millionths of a millimetre 

 shorter. 



Among the important investigations of Dr Konig was one in 1883 to ascertain 

 whether there was any real distinction between the two classes, as he considered their 

 names were no longer justifiable. He had an idea that the position of their neutral 

 point, separating the two colours, might throw light on this, and examined 13 persons, 

 6 red- and 7 green-blind. Their neutral points varied fromw.l. 4917 to 504"75. but were 

 so mixed up as to show no sharp division. He afterwards, however, came to the conclusion 

 that this point was too indistinct and uncertain to form a test, and in 1886 he explained 

 why it could give no certain indications of the two types. 



Professor Stilling' s work of 1883 is largely devoted to the variations which he 

 admits abound in dichromic vision, and which he explains simply by variations in 

 sensitiveness to the red and green rays. See " Data," letter E. The book is illustrated 

 with coloured plates, intended to give an approximate idea of how the spectrum appears 

 to the colour-blind. 



The judgment of Professor Holmgren on the variations in dichromic vision is very 

 valuable, as he is a strong supporter of Young's theory, and particularly as he has devised 

 the means most frequently adopted for distinguishing the two classes. But, as already 

 stated on page 455, he does not admit the old application of the theory, and conse- 

 quently does not consider the ordinary names of " red-blind " and " green-blind " to have 

 the meaning that would seem to be attached to them. A clear statement of his views on 

 this point will be found in " Data," letter S. 



The investigations of Konig and Dieterici, in 1886, comprehended a repetition, 

 somewhat more elaborate, of the comparison undertaken by M. Van der Weyde on the 

 intensity curves of the red-blind and green-blind respectively. Some particulars of their 

 results will be found in " Data," AC. The various determinations of these curves 

 agree fairly well. The blue curve appears to be the same in both classes ; but the yellow 

 curve has two positions, which I have represented, in a simplified shape, in a diagram 

 given in "Data," letter T. The left-hand or longer-waved curve is that for the "green- 

 blind," the right-hand or shorter-waved one is that for the " red-blind ; " — and it will 

 be seen there is a difference of about 30 millionths of a millimetre wave-length in 

 their positions. That means that the red-blind warm sensation would be excited to a 

 given degree by waves a little shorter than those required for the green-blind. This 

 difference of position has an important bearing on the explanation of the phenomena. 



I may now say something of my own observations on this point, made in 1858-59, 

 but first published in the Phil. Mag., July 1892. They consist of several sets of experi- 

 ments by Maxwell's colour-top on cases of colour-blindness, all dichromic, but presenting 

 a wide extent of variation, the exact nature of which is well exhibited by the admirable 

 test employed. The actual equations are given ; a summary of results is collected in a 

 table, and diagrams of the two extreme cases are drawn. I have given in "Data," 



