COLOR BLINDNESS. 197 



is incapable after a time, of distinctly perceiving it; while it retains its 

 full power of perception in reference to a Iresh 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 considered as a compound of red and green, and when the per- 

 ception oi' the red is destroyed, the green must become visible. This 

 explanation, however well it may apply to some of the phenomena, is 

 not sufficient for the whole. Accidental 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 will be perceived 

 which passes in succession througli all the colors of the rainbow. 



Of the real cause of these appearances we are as yet almost entirely 

 ignorant. Professor Plateau, of Ghent, has indeed referred them all to a 

 few simple principles, but these appear to us rather expressions of the 

 law of succession of the phenomena, than physical explanations of them. 

 Wedo not however at this time intend to dwell on this class of phenomena, 

 but to give a succinct account of those peculiarities of vision, in which 

 abnormal perceptions of color are permanent, and which are fully treated 

 of in the memoirs, the titles of which stand at the head of this article. 



The peculiarity of vision called colorblindness, and sometimes Dalton- 

 ism, may generally be referred to two classes. 1. Those in which all im- 

 pression of color, except white and black, are wanting. 2. Those in 

 which the individual can perceive certain simple colors, but is not able 

 properly to distinguish between them. There are persons, strange as it 

 may appear, in whom the sense of primary color is entirely deficient, 

 and who, in place of red, yellow and blue, see nothing but different 

 degrees of white and black. Professor Wartmann gives a number of 

 cases of this kind. The most ancient of those he finds described, is that 

 by Dr. Tuberville, in 1G84, of a woman, of about 32 years of age, who 

 came to consult the Dr. about her sight, which, though excellent in other 

 respects, gave her no impression in reference to color, except white and 

 black. Spurzheim mentions a family, all the membersof which could only 

 distinguish different shades of white and black An account is given by 

 Mr. Huddart of a shoemaker, in Cumberland, who could distinguish in dif- 

 ferent colors only a greater or less intensity of light, calling all bright 

 tints white and alldull ones black. His peculiarity of vision was unknown 

 to him until oneday, while a boy, playing in the street, he found a stocking, 

 and for the first time, was struck with the fact that it was called by his 

 companions red, whereas to his mind it was capable of no farther descrip- 

 tion than that designated by the word stocking; he was thus led to con- 

 clude that there was something else besides the form and position in the 

 leaves and fruit of a cherry tree, perceived by his play mates but not seen by 

 himself. Two of his brothers had the same imperfection, while two other 

 brothersjhis sisters, and other relatives, had the usual condition of vision. 



Of the other class, the cases are much more numerous ; we shall, how- 

 ever, give only a few examples. Mr. Harvey, of Plymouth, mentions a 



