1460 



VISION'. 



advanced by Dal ton and supported with the 

 ability for which he was remarkable, was 

 this : " It appears therefore almost beyond 

 a doubt that one of the humours of rny eye 

 and of the eyes of my fellows, is a coloured 

 medium, probably some modification of blue. 

 I suppose it must be the vitreous humour : 

 otherwise I apprehend it might be discovered 

 by inspection, which has not been done/' * 

 Before his death, the great chemist had re- 

 quested bis friend Dr. Kansome to examine 

 his eyes after his demise : this was done most 

 carefully, and it was ascertained that though 

 the crystalline lens had the slight yellow tinge 

 common in old persons, the vitreous humour 

 of both eyes was absolutely colourless. 

 Nevertheless the hypothesis has been revived 

 by Dr. Trinehinetti-f-, who considers that the 

 defect is produced by a coloration of one or 

 more of the transparent media of the eye, and 

 probably of the crystalline lens, and even goes 

 so far as to advise depression or extraction 

 of the lens as a means of radical cure ! 



GoetheJ attributed the confusion of colours 

 to an insensibility to blue ; whereas Szokalski 

 expressly states that among more than sixty 

 cases of achromatopsy, of which he had notes, 

 there was not one in which there was abso- 

 lute deficiency of the perception of blue. 

 Others again have supposed that the retina 

 itself has a blueish tinge in such cases, whilst 

 Dr. Thomas Young |i attributed achromatopsy 

 to the absence or paralysis of those fibres of 

 the retina which are calculated to perceive 

 red. 



The able metaphysician Dugald Stewart ^T, 

 has viewed the subject from a different point. 

 " In the power (says he) of conceiving colours, 

 too, there are striking differences among 

 individuals. And indeed I am inclined to 

 suspect that in the greater number of in- 

 stances the supposed defects of sight in this 

 respect ought to be ascribed rather to a 

 defect in the power of conception. One 

 thing is certain, that we often see men who 

 are perfectly sensible of the difference be- 

 tween two colours when they are presented 

 to them, who cannot give names to these 

 colours with confidence when they see them 

 apart, and are perhaps apt to confound the 

 one with the other. Such men, it should 

 seem, feel the sensation of colour like other 

 rr.en when the object is present, but are 

 incapable (probably in consequence of some 

 early habit of inattention) to conceive the 

 sensation distinctly when the object is re- 

 moved." 



This explanation would have weight sup- 

 posing persons had rare opportunities of 

 contrasting colours, for then their memory 



* Mem. of the Literary and Phil. Soc. of Man- 

 chester, p. 43. 



t Atti della sesta Riunione degli Scienziati Itali- 

 ani, p. 712. 



t Zur Farbenlehre, 111 to 113. 



Annales d'Oculistique, torn. iii. p. 7. 



|| Phil. Trans, vol. Ixxvi. p. 344. 



*([ Elements of the Philosophy of the Human 

 Miud, p. 73. 



might fail them, and they might feel uncer* 

 tainty as to the proper appellation to be 

 given to a tint ; but if there be one sense 

 more than another which enjoys unbounded 

 licence, it is that of sight ; not an hour 

 passes that the property of distinguishing 

 colours is not called into exercise, and in this 

 age of high civilKation it is in perpetual ac- 

 tivity. Nevertheless Dr. Himly has adopted 

 the same view. It is on record however 

 that intelligent persons have expressly de- 

 clared that their infirmity arose from no care- 

 lessness on their part, as they had made 

 many earnest endeavours to correct it.* 



By phrenologists, achromatopsy has been 

 attributed to imperfect development of the 

 organ of colour ; Szokalski, though believ- 

 ing that there exists in the brain a portion 

 which presides over the function of vision, and 

 that this portion is diminished in volume in 

 persons affected with achromatopsy, adds, 

 *' We know well that phrenologists place the 

 organ of colour in the middle of the super- 

 ciliary arch ; we have, however, examined 

 scrupulously and with great pains many 

 persons who have presented very decided 

 depressions of this arch ; but despite of the 

 best wishes in the world, we could never 

 discover in them any trace of chromatop- 

 seudopsy."f 



Neither does our own experience support 

 the theory of the phrenologists ; in two well- 

 marked Daltonians examined by us, the whole 

 superciliary region was remarkably well de- 

 veloped. 



The vision of the late Mr. Troughton was 

 carefully investigated by Sir John Herschel, 

 who instituted a series of ingenious experi- 

 ments for the purpose of ascertaining, if 

 possible, the cause of the imperfection. 

 The result at which he arrived was, " that 

 ail the prismatic rays have the power of 

 exciting and affecting the eyes with the sensa- 

 tion of light, and producing distinct vision, 

 so that the defect arises from no insensibility 

 of the retina to rays of any particular refran- 

 gibiiity, nor to any colouring matter in the 

 humours of the eyes preventing certain rays 

 from reaching the retina, but from a defect 

 in the sensorium by which it is rendered in- 

 capable of appreciating exactly those dif- 

 ferences between rays on which their colour 

 depends. J 



Hart man n$ is of opinion that it is by ana- 

 lysis that we arrive at a knowledge of ob- 

 jects which present themselves to our notice ; 

 he supposes that we do not perceive them 

 instantly, but little by little and only by exa- 

 mination of their distance, form and colour, 

 which scrutiny rests on a series of changes 

 operating on the retina, ciliary nerves, 

 and motor ocular nerves : we do not easily 

 recognize objects unless this succession of 

 modifications has become babitual and takes 



* See GlasgowMedi ca l Journal, vol. ii. p. 17 ; Op. 

 it. p. 100. 



t Op. cit. p. 100. 



J Encyclopaedia Metropolitan a, article " Light." 



Der Geist des Menschen, S. 152. 1820. 





