40 COLOUR-SIGHT AND COLOUR-BLINDNESS 



It frequently appears that not hue nor colour bufc 

 intensity — brightness or shade — influences the choice ; that 

 skeins totally different in colour, but somewhat the same in 

 depth are chosen. 



Remembering the photo-chemico-vital action of light on 

 the visual purple or rhodopsin and the photo-chemical action 

 of light on the photographic gelatine plate, I experimented with 

 the railway red and green flags. I got some official bunting, 

 samples of both colours and photographed them first on a 

 white background and then on a black. The results are thrown 

 on the screen direct from the negative, for some of the contrast 

 might have been exaggerated if a lantern slide positive had been 

 used. With an Ilford's ordinary plate the actinic exertion of 

 neither red nor green is much ; what there is, is distinctly in 

 favour of green, but considering the intense differences 

 suggested by the two flags photographic results show no great 

 contrast. With Ilford's chromatic plates the difference is 

 much more apparent. These, the chromatic or correct-colour- 

 value plates might be likened to the normal sighted, while the 

 ordinary plates would represent the partially colour-blind. 



By throwing a red light on the screen from a red glass 

 which cuts off the green rays of the spectrum the audience 

 becomes temporarily green-blind and is supposed to see things 

 as the green-blind are suspected to see them ; while by using a 

 blue-green light which absorbs all the red rays the audience, 

 deprived of red, is temporarily red-blind and sees as it is fan- 

 cied the red blind should see. 



Abney is responsible for this experiment but I doubt its 

 truth. With the red light the green flag certainly reflects 

 no colour and looks black, but the red is depraved ; and with the 

 green light the red flag is black and the green is hardly recog 

 nisable. The experiment proves too much ; in each instance it 

 destroys one colour and materially disturbs the other. 



Some time ago I thought I was in the fair way of solving 

 the colour-blind problem. Those of you who photograph must 

 remember an old recommendation, namely, to view your intended 

 landscape picture throi "h deep blue glass so as to get your 

 colour values. Doing this it struck me how very identical in 

 colour was a red brick wall with the green leaves of the creeper 

 that clung to it, and working the experiment out I found that a 

 blue vision would fairly account for such of the colour-blind 

 blunders as I was acquainted with. I turned to all the available 



