ABNORMAL VISION.
In the consideration of abnormal vision, we propose to divide the subject into 1. Abnormal Vision resulting from delft• tive action of the retina or sensorium ; as Achromatopsy, Hyperchronzatopsy, and Anorthopia.
2. Abnormal Vision arising from faulty con figuration of the eye, or from changes in the refractive media ; as Myopia, Pres byopia, and Cylindrical Eye.
Anzaurosis, Chrupsia, and other morbid conditions, do not fall within the scope of this article.
Achromatopay (a not, xprIlaa colour, Sny the eye), or insensibility of the eye to colours, is an affection which has been recognised nearly two hundred years ; but, although cases have been from time to time published in the Phi losophical Transactions and other scientific works, our knowledge of the phenomena of this singular condition is of recent date, and is chiefly due to the labours of Wartmann, Seebeck, Szokalski, Purkinje, Himly, &c.
Various names have been proposed for this imperfection of vision ; but the majority are exceedingly unmanageable. By Sommer and Szokalski the term cliromato-pseudopsis has been employed ; Goethe proposed to call it akyano-blepsis, whilst Purkinje divided the disorder into four varieties,—achroinatopsis, chromato-dysopsis, akyano-blepsis, and anery thro-bkpsis ; others again have been satisfied with the simple term chromato-nietablepsis. Jiingken employs indifferently the denomina tions of achromatopsy, chromatopseudopsy, and chromatometablepsy. Many writers, however, have adopted the term Daltonisnz, proposed by Prevost, and supported by Wartmann ; and, although objectionable as perpetuating the in firmity of an individual, it has the merit of simplicity and easy inflection. The term achromatopsy is, perhaps, that most usually employed, although, strictly speaking, it is only applicable to one class. Still, being ex tensively recognised, we shall adopt it to de signate this imperfection of vision ; occasion ally the expression "Daltonian " may be used for the sake of brevity.
Modifications of insensibility to colours exist in every degree, and in the minor shades are so frequent as to be almost proverbial with reference to the male sex ; indeed, if twelve men, taken at random, were shown a nunmer of ribands of the more delicate colours, a diversity of opinion would almost certainly arise as to the appropriate names.
The number of persons who cannot distinguish certain colours is considerable; but the defect is seldom known to others, those who are con scious of their imperfection being desirous of concealing it, and some perhaps, not being aware of it till accident leads to its discovery. In every case that has fallen under our notice, there has been reluctance to submit to ex amination, from the fear of ridicule ; and indeed it is difficult to repress a smile when a person is seen to match green and scarlet to gether, or crimson and dark blue, and earnestly protest that the colours are absolutely iden tical. The nearest approach to this condition in the healthy eye, and a test by which the embarrasment of Daltonians may be judged of, is the difficulty of distinguishing between blue and green by candlelight; a difficulty which every one must have experienced. Analogous to this defect of the organ of vision, is that better known, because more evident, insensi bility of the organ of hearing, whereby many persons are utterly unable to detect the differ ences between musical notes, or, as pointed out by Dr.Wollaston, their ears may be abso lutely insensible to sounds at one extremity of the scale. Sir David Brewster remarks* that, although his own hearing is perfect and each ear equally acute for all ordinary sounds, yet one of them is absolutely deaf to the chirp of the cricket, while the other hears it distinctly. Dr. Pliny Earle has published t a remarkable illustration of the imperfection of the two senses in conjunction. " The whole family (says he) of which the chart has been ex hibited, is probably no less generally charac terised by a defective musical ear than an imperfect appreciation of colours. Several of the individuals comprised in it are utterly inca pable of distinguishing one tune from another.