Itis very common to meet with eyes differing in their focal leiv,th, and in such cases one usually falls into disuse ; it is also occasion idly found that one eye will be myopic and the other presbyopic, a condition of vision embarrassing to the patient and the sur geon ; but the nature of which may easily be ascertained by careful trial with glasses. Serre, in a memoir on the application of phosphene:, or the luminous spectra excited by compression of the eye*, states that these as sist in the diagnosis of myopia. He says that in such cases the nasal and orbital phos plienes appear equal to the temporal in con stancy, brilliancy, and sometimes in size ; but he cautions us against concluding that the re time of myopes are more sensitive than those of persons having normal vision, ascribing the above appearances to the greater prominence of the eyes, which facilitates compression, and admits of its application to the deeper parts of the eye. These results lie adds, are at com plete variance with the opinion of M. Steeber of Strasburg, who thinks that the proximal cause of myopia may sometimes be a peculiar alteration of the retina admitting of a compli cation of myopia with amblyopia, or com mencing amaurosis ; for that if the retina be essentially affected, the orbital, and sometimes the-nasal phosphenes are scarcely appreciable, or they may be entirely wanting. Dr. Serre further asserts that which is certainly Con trary to received opinions, and (we may add), to our own experience, that when the foci of two myopic eyes differ, the retina of the eye with the shortest focus being that least used, is absolutely stronger and more active than the retina of the other eye.
Myopia May be symptomatic of hydro phthalmia, and of conical cornea. In the former case it will be accompanied with enlargement, and symptoms indicating some inflammation of the eye. In the latter, the change of form in the cornea will be apparent on close exa mination.* Instances have also come under our notice of congenital cataract having been mistaken for myoi.ia. t In such cases the patients have dull vision, and are unable to see small objects distinctly even with the aid of glasses. On careful examination a greyish hue may be detected in the pupils ; and if these be dilated by atropine, the nature of the case is at once displayed by the semi-opaque or slightly turbid lenses be coming visible.
The progress of myopii depends very much upon the line of conduct pursued by the indi vidual. If that be judicious from the com mencement, the defect may not increase, or if acquired, may diminish ; but its march is too often as follows : — A person who is occupied for months together in reading small type (for example), finds that he discerns distant objects less and less distinctly, and at once jumps to the conclusion that spectacles are required ; he goes to an optician, tries a pair, sees better, purchases them, and proceeds with his labour. After a time he finds that these
glasses do not afford the same amount of assistance they did at first ; they are care fully wiped, but -still are not satisfactory, and therefore a pair of a higher power are purchased ; -with these he sees sharply at first, though they do diminish a little ; but they in time are laid aside, and the individual goes on and on, until the whole scale has been run through, and he is half blinded for the remainder of his life. In the progress of such a case other symptoms often develop them selves ; not only is the sight rapidly shortened; but it is weakened; the eyes ache, and are speedily fatigued by application ; there is op pression about the brow, and often headach; he is teased with muscm volitantes, and the sight is obscured from time to time by a mist.
The case now assumes the character of impaired vision from overwork, and unless judicious treatment be adopted, amaurosis may be the result.
It is -important to know that near sight may be acquired in childhood by the common practice that children have, of approaching their eyes very close to any object on which their attention may be engaged. When learn ing to read or write, or draw, they almost invariably hold their faces sideways, nearly touching the slate or paper. This should at all times be discouraged. In like manner infants should not have very small toys given them, or such as require to be looked at closely, for not only may they be rendered short-sighted, but strabismus may be caused, for as the visual axes naturally converge when objects are held very near the eyes, the frequent repetition of this may occasion a squint. For the same reason there is sound judgment in printing children's books in good bold type, and it is well to encourage young people to observe distant objects, and to describe what they see in trees, landscapes, the exterior of houses, &c. As a general rule glasses are not ad missible for young persons, for if worn there is scarcely any hope c f amendment ; but it is sometimes necessary to allow them during music lessons, in which case they should be of the lowest power that enables the child to sec the notes, and their use should be re stricted to that pursuit.
Myopic persons, generally, should remove ohjects they are regarding as far from them as possible, and should avoid small type, minute writing, and microscopical inves tigations. The use of a high desk is very important, not merely as tending to prevent oculo-cerebral congestion, but also pain in the chest, which is often caused by stooping ; and as a general rule, all near-sighted persons, of whatever age, should exercise the eyes, when in the open air, by endeavouring to make out distant objects.