Artificial Colour-blindness hy Moonlight. 



217 



When green-blindness is induced by exposure of the eye to intense 

 green light, not only is the observer unable to perceive the colour of 

 green objects, but the sensation of green is no longer excited by the 

 intense green light that caused the blindness. And the same may be said 

 of blue-blindness. On the other hand, with artificial red-blindness the 

 exciting light still looks reddish, though greatly dulled and much paler 

 in hue, but all objects less brightly illuminated fail to excite the red 

 sensation. Probably in the case of green-blindness the green sensation 

 is not entirely destroyed, but reduced so much that the red and blue 

 sensations, which are also excited by that same part of the spectrum, 

 completely overpower it. In producing red-blindness, as there are no 

 colours to the left of red, I have generally used a part of the spectrum 

 which excites only the red sensation, and which therefore must con- 

 tinue to appear red if visible at all. But I have not thought it desir 

 able to push the fatigue of the retina far enough to destroy the sensation 

 of light. 



For the mere demonstration of the phenomena of colour-blindness, 

 light of quite moderate intensity is amply sufficient if the precaution is 

 taken of shielding the eye from all other light during the experiment, 

 and of giving it time to recover from the effects of previous illumina- 

 tion. The colour-blindness so produced is, however, not absolute, but 

 merely relative, the sensation which has been fatigued, whether red, 

 green, blue, or violet, being still excited by a stronger stimulus. 



The following is perhaps the most striking and suggestive way of 

 making the experiment : — 



1. I exposed my left eye to direct moonlight in the focus of a lens 

 behind a screen of ruby glass combined with a gelatine film stained 

 with magenta. After three minutes I looked through a spectroscope 

 directed to the moon. The red had entirely disappeared, and only the 

 green, blue, and violet were visible. With the right eye I could see 

 the red as well as the other colours. 



2. I exposed my right eye in the same manner to moonlight, using a 

 screen of green glass instead of the red. On looking through the spec- 

 troscope I found the green sensation had entirely vanished, the red 

 meeting the blue in the same part of the spectrum, viz., between E and 

 h, as in my experiments with sunlight.* The violet was easily distin- 

 guished from the blue in this case also. The left eye was still partially 

 red-blind, and the contrast between the spectrum as seen by it and by 

 the right eye was very marked. 



I was unable to use spectral colours for fatiguing the eye because the 

 full moon is not visible at this season of the year from the laboratory 

 in which the large spectroscope is mounted, and the intensity of the 

 light was too much reduced by reflection from the two mirrors of the 



* ' Phil. Trans.,' B, vol. 191, Plate I, figs. 4 and 5. 



