Prof. Tyndall on a peculiar case of Colour Blindness. 331 



Duboscq's lamp. A portion of the light was permitted to pass 

 through a bright green glass and was received upon a screen ; 

 no change of colour was perceived : the space on which the green 

 light fell was merely a little less intensely illuminated than the 

 remaining portion of the screen. Lycopodium was shaken upon 

 glass : the electric light looked at through such a glass gives, as is 

 known, a series of brilliantly coloured rings : to Captain C, how- 

 ever, no colour was manifest, merely light and obscure rings fol- 

 lowing each other in succession. A spectrum was cast upon the 

 screen in which all the prismatic colours shone vividly ; to Cap- 

 tain C. only two colours were manifest, namely blue and whitish- 

 yellow. The outline of the spectrum was the same to him as to 

 me ; all that gave me light gave him light also ; but in his case 

 the red, orange, and green were so modified as to produce the 

 uniform impression of whitish-yellow. In some cases of colour 

 blindness, where the sufferer confounds red with green, it is diffi- 

 cult to say whether he takes the red for a green or the green 

 for a red. In the present case neither of these expresses the 

 fact ; neither red nor green is seen^ but both of them are reduced 

 to a colour different from either. 



Captain C. assured me, that, previous to the circumstance re- 

 lated at the commencement, he was a good judge of colours, so 

 that in pronouncing upon any colour he has an aid from memory 

 not usually possessed by the colour blind. Indeed I had myself 

 an opportunity of reviving his impression of red. A glass of this 

 colour was placed before his eyes while he stood close to the 

 electric lamp : on establishing the light, he at once exclaimed, 

 " that is red ! " He appeared greatly delighted to renew his 

 acquaintance with this colour, and declared that he had not seen it 

 for several years. The glass was then held near the light while 

 he went to a distance, but in this case no colour was manifest; 

 neither was any colour seen when a gas-lamp was regarded 

 through the same glass. The intense action due to proximity 

 to the electric light appeared necessary to produce the effect. 

 " You gave the eye a dram,'^ observed a gentleman to whom I 

 described the case : the figure appears to be a correct one. 

 Captain C.'s interest in this experiment was increased by the 

 fact, that the Portland light, which he has occasion to observe, 

 has been recently changed from green to red, but he has not 

 been able to recognize this change. The fare in the fore-cabin 

 of a vessel of his own which he now commands happens to be 

 sixpence, and he is often reminded by the passengers that he has 

 not returned their change. The reason is, that he confounds a 

 sixpence with a half-sovereign, both being to him of the same 

 colour. A short time ago he gave a sovereign to a waterman, 

 believing it to be a shilling. 



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