COLOR BLINDNESS.* 



By Joseph Henry. 



[Fiom the Princeton Review, for July, 1845.J 



It is an interesting fact in reference to the dependence of one class, 

 at least of onr knowledge, on sensation, that many persons are born with 

 defective vision and yet remain for years of their lives without being 

 conscious of the deticieucy. We know a gentleman who had probably 

 been always near sighted, but who did not discover the peculiarity of 

 his vision until the age of twenty-five, when it was accidentally made 

 known by looking at a distant object through a concave lens. Many 

 persons whose eyes are sound and capable of exercising the most deli- 

 cate functions, are permanently unable to distinguish certain colors. 

 And the number of such persons is much more considerable than we 

 would be led to imagine Irom the little attention this defect of vision 

 has excited. It is often unknown to the individual himself, and indeed 

 only becomes revealed by comparing his powers of discriminating dif- 

 ferent colours with those of other persons. The eye also under some cir- 

 cumstances may lose its sensibility for particular colors, or be thrown 

 into such an unusual state as to present all objects to the mind under 

 the appearance of a false color. Thus if a person looks fixedly for a time 

 at a bright red object and then turns his eye to a white wall, he will 

 perceive a green image of the red object depicted on the white surface. 

 A lady of our acquaintance was once thrown into an alarming but laugh- 

 able i)aroxysm of terror by an eflect of this kind. She had been tor some 

 hours attentively sewing on a bright crimson dress, when her attention 

 was directed towards her child, who, in its sport, had thrown itself on 

 the carpet; its face appeared of the most ghastly hue, and the affrighted 

 mother screamed in agony, that her child was in convulsions — the other 

 inmates of the house hastened to her assistance, but they were surprised 

 to find the little one smiling in perfect health. The sanity of the mother 

 became the natural object of solicitude, until the effect was properly re- 

 ferre<l to the impression made on her eye by the crimson cloth. 



Phenomena of this kind are known by the name of accidental colors; 

 they have long attracted the attention of the natural philosopher, but 

 the explanation of them is still involved in considerable uncertainty. 

 The hypothesis which has been most generally adopted is that the eye 

 by long attention to a particular color, becomes fatigued with this and 



* 1. Observations on colour bliuclness, or insensibility to the impression of certain 

 colours. By Sir David Brewster, K. H., &c. Philosophical Magazine. 



2. Memoir on Daltonism, (or colour blindness.) By M. Elie Wartmann, Professor of 

 Natural Philosophy in the Academy of Lausanne, &c. Scientific Memoirs. 



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