282 SENSE OF SIGHT. 



. 



rous cases of achromatopsia, as this defect has been termed, and 

 has added the history of one which fell under his own care, was led to 

 infer, from all his researches : 1, that entire inability of distinguishing 

 colours may co-exist with perfect ability to perceive the forms of objects ; 

 2, that the defect may extend to all but one colour, and in such case 

 the colour recognised is always yellow ; and, 3, that the defect may 

 extend to all but two colours, and in such case the colours recognised 

 are always yellow and blue ; yet that this is not the fact is sufficiently 

 shown by the examples already given. Dr. Pliny Earle 1 has referred 

 to a number of cases, which came within his knowledge, and most of 

 them under his own observation, in which the inability would seem to 

 have been hereditary. Dr. Earle's maternal grandfather and two of 

 his brothers were characterized by it ; and among the descendants of 

 the first mentioned, it is met with in seventeen. When thus entailed, 

 it would appear to overleap, at times, one generation or more. It would 

 appear, too, that males are more frequently affected than females. Dr. 

 Earle observed, that the power of accurately distinguishing colours 

 varies at different times in the same person ; and that it is not unfre- 

 quently connected with, or accompanied by, a defect in the power of 

 discriminating musical tones. 



The opinions of philosophers have varied regarding the cause of 

 this singular defect in eyes otherwise sound, and capable of performing 

 every other function of vision in the most delicate and accurate man- 

 ner. By some, it has been presumed to arise from a deficiency in the 

 visual organ; and by such as consider the ear to be defective in func- 

 tion in those that are incapable of appreciating musical tones, this de- 

 ficiency in the eye is conceived to be of an analogous nature ; and the 

 analogy is farther exhibited by the facts, just mentioned, observed by 

 Dr. Earle. " In the sense of vision," says Dr. Brown, 2 " there is a 

 species of defect very analogous to the want of musical ear, a defect 

 which consists in the difficulty, or rather the incapacity, of distinguish- 

 ing some colours from each other and colours which, to general 

 observers, seem of a very opposite kind. As the .want of musical ear 

 implies no general defect of mere quickness of hearing, this visual 

 defect, in like manner, is to be found in persons who are yet capable 

 of distinguishing, with perfect accuracy, the form, and the greater or 

 less brilliancy of the coloured object ; and I may remark, too, in con- 

 firmation of the opinion, that the want of musical tone depends on 

 causes not mental but organic, that in this analogous case some at- 

 tempts, not absolutely unsuccessful, have been made to explain the 

 apparent confusion of colours by certain peculiarities of the external 

 organ of sight." 



Dr. Dalton, who believed the affection to be seated in the physical 

 part of the organ, has endeavoured to explain his own case, by sup- 

 posing, that the vitreous humour is blue, and therefore absorbs a great 

 portion of the red and other least refrangible rays ; and Sir David 

 Brewster, in the "Library of Useful Knowledge," 3 appears to think 



1 American Journal of the Medical Sciences, April, 1845, p. 346. 



2 Lectures on the Philosophy of the Human Mind, vol. i., Boston, 1826. 



3 Natural Philosophy, vol. i., Optics, p. 50, Lond., 1829. 



