Presbyopia, or Far-sightedness (from Tplal3os, " old," and ;10, " eye ;" this being a state of vision to which old age is almost invariably subject.) Long-sightedness, as the name sufficiently indicates, is an affection the reverse of the one just described, and depends upon opposite causes. Either the refractive powers of the eye are too feeble, or its axis is shorter than is natural ; the result is an imperfectly formed image on the retina, from the rays of light not converging sufficiently soon to be brought to a focus. Hence the far-sighted person removes the object be is examining farther from him, or he makes use of glasses whose effect is to increase the refraction of the rays of light before they enter his eye. Far-sightedness is sometimes met with in the young ; but it is rare that an individual lives to be old without be coming presbyopie indeed the sinking of the eyeballs, the flattening of the corneae, and the smallness of the pupils, all which contribute to this effect, are among the series of changes which every part of our body undergoes as we advance in age. The time of life at which pres byopia first shows itself is generally about forty-five; but there arc great differences in this respect, some persons requiring the use of convex glasses at thirty as much as others at fifty. Among the earliest symptoms observed, are a difficulty of reading small print, in nibbing a pen, or in examining small objects ; the letters of a book appear misty, and run one into another ; and if the effort is long continued, the eyes become fatigued and the head aches. Notwithstanding this difficulty of distinguishing near objects, distant ones continue to be seen as clearly as before.
Presbyopia, after it has once appeared, generally goes on increasing, so that an individual thus affected requires to change his glasses from time to time for those of a higher power : instances, however, are recorded of old persons long accustomed to the use of convex glasses recovering their former sight at the age of 80 or 90 years.
Treatment.—The same principles that we have laid down for the treatment of myopia, and for the use of concave glasses, are applicable, regard being bad to the opposite condition of the eye, to the present affection. Convex glasses should not be had recourse to too soon, nor should too high a power be used, but the lowest that answers the purpose is to be chosen. When preabyopia occurs suddenly, and in an individual much below the age at which It ordinarily occurs, there is some mischief to be suspected either in the eye or in the brain, which will require an antiphlogistic treatment and a total suspeusien of the use of the eyes in regarding near or small objects.
Double Vision, Visas duplicatus, or Diplopia, may arise either from a want of correspondence in ,the movements or position of the two eyes, the vision of each eye singly being perfect ; or there may be double vision with one eye only, while the harmony in the movements of the two is not disturbed. The most common example of the first form of the affection is afforded by eases of squinting; but as this defect is treated of in a separate article, we merely allude to it in this place. More serious and less common is the loss of harmony in the movements of the eyes which results from paralysis of one or more of the orbital muscles. If one muscle only is affected, the eye will move in harmony with its fellow in every directiou but the one towards which its paralysed muscle should draw it, consequently in this direc tion objects will be seen double ; but if several are affected, as is not unfrequently the case, then the:movementa of the eye will be still more restricted, and there will be single vision only when the axis of the sound eye is parallel with that of the paralysed one. These affections appear sometimes to arise from cold ; at other times they are depen dent upon disease about the base of the brain, as some tumour pressing on the motor oculi nerve, or there may be an inflammatory condition of the brain and its membranes, or a sanguineous or serous effusion involving the origin of the third pair of nerves. Whichever of these
may be the cause, our treatment must be directed to remove it, while the state of the eye will be an index of the success or failure of the remedies we make use of.
Double vision with a single eye is a more rare affection than the oue just described, and depends upon some irregular refraction of the cornea or lens.
M. Pr6veat, who published an account of his own case in the An 'sales de Chimie et de Physique,' 1832, thought it might arise from a fracture, bruise, or partial flattening of the lens, or separation of its Laminae. I'rofossor Airy and Mr, Babhage are troubled with this defect, the latter gentleman with both eyes, but he is able to remedy it by looking through a small bole in a card, or through a concave lens. Professor Airy finds that his eye refracts the rays to a nearer focus in the vertical than in the horizontal plane, and he has ingeniously con trived to remedy it by the use of a double concave lens, one surface of which is spherical and the other cylindrical. The spherical surface is to correct the general defect of a too convex cornea ; the cylindrical is to converge or diverge those rays at right angles to the axis, while the parallelism of those which impinge upon it in the plane of its axis is unaffected. Thus the focus of the spherical sUrface will remain unaltered in one plane, but in the other it will be changed to that of a lens formed by it and a spherical surface of equal curvature with the cylinder. With the aid of a glass of this description Professor Airy could read the smallest print at a considerable distance equally well with the defective as with the sound eye. He found that vision was most distinct when the glass was pretty close to the eye and the cylindrical surface turned from it. " With these precautions," be observes, " I find that the eye which I once feared would become quite useless, can be used in almost every respect as well as the other." Colour Blindness ; Dichromisin ; Chromatopseudopsis ; Defect of the Sense of Colour ; Daltonism.—There are some persons who, although they can see the size and form of objects perfectly well, have neverthe less a deficient power of distinguishing colour. Many distinguished individuals have been subject to this : such as Dugald Stewart, M. Sismondi, and John Dalton, the celebrated chemist. The subject first excited general attention by the latter distinguished individual having reported his own case, and the deficiency was hence called Daltonism. This subject was investigated by the late Dr. George Wilson, of Edinburgh, who has written a work on Colour Blindness' (1855), which contains by far the fullest account of this defect which exists. Dr. Wilson divides the colour-blind into three classes. 1. Inability to discern any colour properly so-called, so that black and white, that is, light and shade, are the only variations of tint perceived. 2. Inability to discriminate betweeu the nicer shades of the more com posite colours, such as browns, grays, and neutral tints. 3. Iuability to distinguish betweeu the primary colours—red, blue, and yellow ; or between those and the secondary colours, such as green, purple, orange, brown. The last variety is the most common. In all these cases it would appear that the primary defect is the inability to distinguish the red colour, and the influeuce it exerts on the other colours of the spectrum. Dr. Wilson has narrated a large number of eases, and shown that this derangement of vision is much more common than has hitherto been supposed. He calculates from his own observations that at least one person in fifty is thus affected. Dr. Wilson has shown that it becomes very important to knew where this defect exists, as it incapacitates people for occupations where the appreciation of colour is important. He has specially shown that, in cases of engine-drivers on railways and sailors at sea, who are warned of danger by coloured • signals, this condition may lead to serious disasters.