COLOR-BLINDNESS. 471 



of these or other true spectra, where the two eyeballs have been re- 

 moved, appears to have been recorded; so that it has not been proved 

 that such spectra can occur without the intervention of the retinal 

 elements. Cases of disease of the retina, involving total blindness, 

 are not quite satisfactory as evidence of this, for the retina may still 

 be exceptionally excitable. 



In many persons, the sensibility of the retina to colors is remarkably 

 acute, enabling them to discriminate between shades of the same color, 

 which, to other persons, present no difference. There are others whose 

 sensibility to certain impressions of color, is curiously defective. This 

 affection, known as achromatopsy, color-blindness, or Daltonism, con- 

 sists in an inability to distinguish one color from another. It is more 

 common in the male than in the female, and is often hereditary. In 

 some cases, it is limited to the lighter tertiary tints, which cannot be 

 distinctly recognized ; or there may be inability to distinguish some 

 of the secondary colors ; or the insensibility of the eye to colors may 

 be so great, that one of the primary colors, usually red, may not be 

 recognized, constituting dichromism (Herschel). Thus, bright red is, 

 by some persons, indistinguishable from green ; ripe fruit, such as 

 cherries, being, to them, of the same color as the leaves. Lastly, some 

 individuals can only distinguish black, white, and gray. 



Insensibility of the eye to colors is sometimes a temporary affection 

 dependent on internal causes, such as congestion of the brain, retina, 

 or choroid, or it may be due to a deranged condition of the digestive 

 organs. As to the cause of color-blindness in general, nothing is 

 known ; it probably has its seat in the retina, perhaps in some defi- 

 ciency in the structure or energy of the rods or cones; just as defects 

 in the appreciation! of pitch and timbre, in the hearing of certain 

 individuals, are supposed to depend on defects in the rods of Corti, or 

 of some of the other complex structures of the cochlea. By some, 

 however, the cause of this defect is believed to be in the sensorial 

 nervous centre ; it has also been suggested, that it may be due to a 

 peculiarity in the absorptive property of some of the humors of the 

 eye. Comparatively harmless in most persons, this defect may be of 

 serious consequence in the case of railway guards, pointsmen, or sail- 

 ors, who have to watch signals given by means of colored lights. 



It is by the forms and colors of the retinal images, that we judge 

 visually of the forms and colors of the material world ; but the sense 

 of sight is educated by experience, and by comparison with the results 

 of the tactile sense, and thus, as we have seen, suggests to us complex 

 notions. This fact is illustrated by the cases of persons born blind, 

 who are said at first to imagine that the field of vision is flat, and even 

 that objects touch the eyes. The education of the eye, for distant 

 vision in sailors, for the detection of minute objects by the microscopist, 

 and for the appreciation of form, texture, and color, in various com- 

 mercial and manufacturing pursuits, as well as amongst artists, is well 

 known. There is often a mental, as well as a visual, training in these 

 persons. An acquired acuteness of vision may become hereditary, as 

 would seem to be the case in the Mongols and Hottentots. The sense 



