ACCIDENTS BY RAIL AND SEA. 155 



As is readily observed, green is then precisely the color which, beiog 

 ordinarily the most whitish of the primitive colors, is the first to lose its 

 quality of " saturated" color, and shades into gray. This must then be 

 the exact scheme of pathological color-blindness, according to the theory. 

 In fact, we have found in our examinations a large number of cases per- 

 fectly harmonizing with this scheme. We have, therefore, classed them 

 under the head of incomplete colot^-hlindness ; and this from essentially 

 practical reasons. To define their nature according to the theory, it is 

 necessary to regard them as a particular variety, which we shall call a 

 feeble sense of colors. We are not of course able to decide how far defects 

 of this kind should be considered as having a pathological origin, or 

 whether they are ever congenital. For this determination a much wider 

 experience in this particular department than we now possess is requi- 

 site, for reasons to be given hereafter. This kind of defect in the sense 

 of color leads, if we fancy it carried to its highest degree, or in such a 

 condition that all the elements lose sensibility, to the complete absence 

 of perception of light, that is to say, to blindness, strictly so called. 

 Every defect in the sense of color must then proceed either from a sensi- 

 bility anomalously reduced to a complete paralysis of one or several kinds 

 of elements, or from a number relatively diminished in sensibility to the 

 complete absence of one or several amongst them. 



The experience acquired by an examination of colors in different parts 

 of the visual field elucidates our theory of color-blindness, while at the 

 same time having a practical value. The following is the manner in 

 which this examination is conducted: the eye is fixed upon an im- 

 movable point; a colored object — for example, a colored paper one or 

 two centimetres (two or four-fifths of an inch) square — is slowly 

 passed from the side of the visual field toward the fixed point. This 

 experiment is performed still better by means of a special instrument, 

 Forster's perimeter. We then find that the colored surface, of any color 

 whatsoever, appears completely colorless at the extreme periphery of 

 the visual field. Surfaces of different colors exhibit only variations 

 in intensity of light, not in color. The ground on which the surface 

 appears plays here an important part; since every time our visual 

 sense perceives the light and color of an, object, it partly depends upon 

 the comparison with that which surrounds it. Thus a colored surface 

 seems to us, in this part of the visual field, black or gray on a light 

 ground, and gray or white on a dark. If while following the same 

 direction, the colored object is carried within the region which surrounds 

 the rest of the visual field, like a belt of greater or less width, we begin 

 to see the colored object, but not always in its natural color. Two colors 

 alone, yellow and blue, retain their natural colors. All the others have 

 the appearance of one of these colors, consequently yellow or blue. It 

 is only after the colored object is carried a little farther toward the fixed 

 object that it is seen in its natural color. Consequently we normally 

 see colors only in the middle of our visual field, within a compass extend- 

 ing in a more or less eccentric manner in every direction from the fixed 



