THE EYE. 



All kinds of yellows and blues, except sky-blue, he coulc 

 discern with great nicety. His father, his maternal uncle, one 

 of his sisters and her two sons, had all the same defect. 



A tailor at Plymouth, whose case is described by Mr. Harvey, 

 regarded the solar spectrum as consisting only of yellow and 

 light blue ; and he could distinguish with certainty only yellow, 

 white, and green. He regarded indigo and Prussian blue as 

 black. 



Mr. II. Tucker described the colours of the spectrum as 

 follows : 



Hed mistaken for . 

 Orange ,, 

 Yellow sometimes 

 Green ,, 

 Blue . 



Indigo , , 

 Violet 



brown. 



green. 



orange. 



orange. 



pink. 



purple. 



purple. 





A gentleman in the prime of life, whose case I had occasion to 

 examine, saw only two colours in the spectrum, viz. yellow and 

 blue. When the middle of the red space was absorbed by a blue 

 glass, he saw the black space with what he called the yellow oa 

 each side of it. This defect in the perception of colour was ex- 

 perienced by the late Mr. Dugald Stewart, who could not perceive 

 any difference in the colour of the scarlet fruit of the Siberian 

 crab, and that of its leaves. Dr. Dalton was unable to distinguish 

 blue from pink by daylight ; and in the solar spectrum the red 

 was scarcely visible, the rest of it appearing to consist of two 

 colours. Mr. Troughton had the same defect, and was capable of 

 fully appreciating only blue and yellow colours ; and when he 

 named colours, the names of blue and yellow corresponded to the 

 more and less refrangible rays ; all those which belong to the 

 former exciting the sensation of blueness, and those which belong 

 to the latter the sensation of yellowness. 



In almost all these cases, the different prismatic colours had 

 the power of exciting the sensation of light, and giving a distinct 

 vision, of objects, excepting in the case of Dr. Dalton, who was 

 said to be scarcely able to see the red extremity of the spectrum. 



Dr. Dalton endeavoured to explain this peculiarity of vision by 

 supposing that in his own case the vitreous humour was blue, and 

 therefore absorbed a great portion of the red and other ! 

 refrangible rays ; but this opinion is, we think, not well founded. 

 Sir J. Herschell attributes this state of vision to a defect in the 

 sensorium, by which it is rendered incapable of appreciating 

 exactly those differences between rays on which their colour 

 depends. 

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