Achromatopsy may be the result nf injury, as in the following case related by Dr. Boys de Loury.A' An individual was struck by a pistol ball which entered his mouth without touch ing the tongue and broke through the hard palate and base of the orbit. After his re covery from the wound, the injured eye re tained but little sight; only a small spot of the retina was sensible to light, and to use this, the eye was obliged to be thrown con siderably to one side. Then objects were seen distinctly but without colour. This person described a palette spread with co lours, as a plate with many holes, and con founded the thumb opening with the spots where the colours were placed.
Various explanations, differing widely, have been offered to account for the insensibility of the eyes to colour ; and we may re mark that the subjects of this affection have in several instances been men of intellectual eminence. As for example, the metaphy sician Dugald Stewart, Dr. Dalton the illus trious chemist, Mr. Troughton the eminent optician, Dr. Sommers, Dr. Unzer, of Al tona, and Professor Brandis. The opinion advanced by Dalton 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 my 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 his friend Dr. Itansome 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. Trinchinettit, 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 ! Croethet 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 attributed achromatopsy to the absence or paralysis of those fibres of the retina which are calculated to perceive red.
The able metaphysician Dugald Stewartlf, has viewed the subject from Zdifferent 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 n.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 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 civilisatibn 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."-j
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 all 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 gibility, 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.1: Hartmann§ 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 habitual and takes place easily, hence achromatopsy results from a certain state of torpor and indolence of the retina and motor muscles of the eye ! Professor Wartmann, the most recent au thority upon this subject, stated in his first Memoir that achromatopsy (or Daltonism) has its origin 'in a defect of the sensorium. In his second Memoir he enters at length into a somewhat different explanation of the phenomena. " I admit (says he) with Harvey, Young, Jiingken, Miller, and others that its scant is in the retina, and I think that it is produced by an abnormal state of the nervous expansion, in such sort that it reacts equally under two or more differently coloured vi brations. If the vibration caused by a ray of red is identical with that engendered by a green ray, there will be confusion of these colours. This theory is independent of all systems destined to explain light. * * The theory which explains Daltonism by an ab normal elasticity of the retina has the ad vantage of substituting a reasonable physical condition for a vague notion of the sensorium besides, it is supported by facts, bee:fuse the injuries which alter the ordinary constitution of the visual organ are capable of exciting, permanently or temporarily, a false perception of colours. Lastly it appears to be confirmed by the circumstance that with many Daltonians the eye•sees less distinctly the red rays than those of which the refrangibility is greater. * The actual seat of the phenomena of achro matopsy must, after all, remain matter of speculation, as it is one of those things in capable of demonstration. D'Alembert says: " It is very plain that the word colour does not designate any property of body, but merely a modification of our mind : that, for instance, whiteness and redness exist only in us, and by no means in the bodies to which we refer them by a habit in force from in fancy." The knowledge that we possess of the existence of colours is derived from the evidence afforded by the thousands of persons endowed with the power of distinguishing Them, and therefore we conclude that they do exist. But supposing we were all like Dalton and many others whose visual organs never appreciate red any more than the gene rality of eyes distinguish the calorific or ac tinic rays, we should then not be aware of the existence of the colour called red, which plays so conspicuous a part in the adornment of the universe ; or if some few eyes gifted with superior powers discerned it, the majority would have to admit its existence on the evidence of others, not from knowledge de rived from their own eyes As regards remedial measures, Wartmann and Seebeck recommend the employment of coloured glasses of a certain known tint ; suppose this tint red, the impression of a green body and of a red body at first the same to the naked eye will be distinguished by the use of the transparent screen. Wartmann, however, admits that this me thod only remedies mistakes in the specific nature of colours, and not those which apply to the shades of one and the same tint. Jiingken and Chelius have recommended the use of coloured hands, bearing the name of . their colour, and Szokalski has suggested that sensations of the various shades may be ex cited by fixing the eyes on different coloured patterns, and then on a black or white surface. But this proceeding is scarcely so likely to be productive of benefit as that recommended by Professor Wartmann.