352 Report of the Committee on Colour-Vision. 



colours in accordance with his psychophysical colour-perception, 

 and thus show distinctly to which class he belongs. 



The third principle is that colours may be changed to colour- 

 blind persons whilst leaving them unaltered to the normal 

 sighted. 



Fourthly, the phenomena of simultaneous contrast are much 

 more marked with colour-blind than with normal sighted persons. 

 Two colours not changed to the normal sighted, on being con- 

 trasted, apparently alter considerably to the colour-blind. 



These tests are described in full in my book on Colour-blindness 

 and Colour -perception in the International Scientific Series. 



Question — How would you proceed, supposing you were asked 

 by a railway company to test 500 men ? What arrangements 

 would you make, and how long a time would the examination be 

 likely to occupy? — I should examine each separately, taking 

 care that the others did not look on. I should first examine with 

 the Classification Test, and then put them through the Lantern 

 Test, taking twenty answers in each case. The process would 

 only take about five minutes for each man, with even one 

 examiner, because one man could be going on with the classifica- 

 tion whilst another was being examined with the lantern. I 

 allow ten minutes for each man, because I think that it is a great 

 mistake to hurry, or be in any doubt about a case. 



Question — At that rate it would require 80 or 90 hours to 

 examine the 500 men ? — It would mean a considerable expenditure 

 of time. You want to know if a man can distinguish between 

 red, green, and white at a distance of two miles. You might 

 commit yourself to the Lantern Test alone ; but if one man was 

 sorting the wools while another was being tested with the lantern, 

 it would take little or no longer to employ both tests, and in 

 rejecting a man the double test would be conclusive. I regard 

 both tests as desirable, and the Lantern Test as essential, for 

 that would detect scotoma, whereas the Classification Test would 

 not. 



Question — You said you tried a progressive atrophy case, and 

 that he was a 2-unit man, and his junction at about the E line ? — 

 I cannot say without referring to my note-book, but he saw one 

 part of the spectrum as whitish and the other blue, only seeing 

 those two colours. [Capt. Abney said : — When I tried him the 

 junction of the white was at X 4*733, between F and Gr, and 

 nearer to F, there being a sudden commencement of blue at this 

 point. At 26*5 of my scale he saw a little blue, and at 26*75 no 

 colour.] 



The essential part of my theory is that psychophysical percep- 

 tion is due to the brain and not to the retina. The theory I 

 have formed is that the visual purple is liberated from the rods 

 by light, and forms a photograph at the back of the retina, and 

 that the cones only act as transferring organs of the percipient 

 fibres, transferring the impression of the photograph to the 

 brain. 



