234 WRITINGS OF JOSEPH HENRY. [1845 



The eye also under some circumstances may lose its sensi- 

 bility for particular colors, or be thrown into such an unusual 

 state as to present all objects to the mind under the appear- 

 ance 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 de- 

 picted on the white surface. A lady of our acquaintance 

 w^as once thrown into alarming but laughable paroxysm of 

 terror by an effect of this kind. She had been for 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 sur- 

 prised 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 referred to the impression made 

 on her eye by the crimson cloth. 



Phenomena of this kind are known by the name of acci- 

 dental colors; they have long attracted the attention of the 

 natural philosopher, but the explanation of them is still 

 involved in considerable uncertaint3^ The hypothesis which 

 has been most generally adopted is that the eye by long at- 

 tention to a particular color becomes fatigued with this and 

 is incapable after a time of distinctly perceiving it ; while 

 it retains its full power of perception in reference to a fresh 

 color. The consequence of this is that when the eye is 

 directed to a white surface, after having attentively regarded 

 a red object, green must appear ; because white may be con- 

 sidered as a compound of red and green, and when the per- 

 ception of the red is destroyed, the green must become visi- 

 ble. This explanation, however well it may apply to some 

 of the phenomena, is not sufficient for the whole. Acci- 

 dental colors can be perceived in the eye itself in perfect 

 darkness. This is shown by steadily regarding for a short 

 time a brilliant lamp, and then covering the eyes with the 

 hands so as to exclude all external light, a luminous spot 



