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Affections of the Sense of Sight

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AFFECTIONS OF THE SENSE OF SIGHT.

(Myopia) and (Hypermetropia) have been explained on pp. 454 and 455. For the former the treatment is doubly concave spectacles, for the latter doubly convex spectacles, which any optician can sup ply. Care should be taken never to get concave spectacles too strong, else the short-sighted eye is converted into a long-sighted one. Persons cannot suffer from any degree of short-sight without being aware of it. They cannot recog nize a friend at any distance, nor read a sign over a shop door on the opposite side of the street from them. But long-sighted persons see well at a distance. If they are only slightly long-sighted it is only when they come to look at near objects that the defect occurs. But when the error is slight, persons may not be aware of the long-sight, so effectually does the focussing serve their purpose. But they are continually focussing to see objects which ordinary eyes perceive without any alteration of the lens ; thus their apparatus for accommo dation of the eye (p. 453) is continually in use and becomes strained. The person may go on for years unaware of this. By and by, how ever, the person begins to feel an undue strain on his eyes with reading, writing, drawing, &c. His eyes ache with any prolonged use. They look red and watery after work. After read ing for a time the letters r)In together. He shuts his eyes for a moment(and then on open ing them he can read a little time longer, till the letters again run together, and so on. It is easily seen, therefore, how giving a short sighted person too strong glasses, and convert ing him by them into a long-sighted individual, makes matters worse.

A. short-sighted person is likely to improve as age advances. The lens becomes flattened, does not converge the rays of light so strongly, and therefore the degree of short-sight di. minishes. As years pass, weaker glasses often suffice.

Weak Sight (Asthenopia) is as often due to a slight degree of long-sight as to anything else, and the remedy for this should be tried.

Defective Sight from Age (Preabyopia Greek, present, an old man). With advancing years the lens loses its elasticity, and conse quently its degree of convexity undergoes less change than in youth. It also becomes flatter. Near objects are not so readily focussed. As age grows, persons find themselves less and less able to read and do tine work with the unaided eye. Happily convex spectacles readily restore

the former distinctness of vision, if nothing else is at fault with the eye.

Astigmatism is a curious defect of the eye, first observed by Thomas Young. It is due to the cornea having different degrees of curvature in different directions. Usually the vertical meridian is more curved than the horizontal. The result is that rays of light passing through the vertical meridian will come to a focus sooner than rays passing through the horizontal meridian. Consequently, all the rays from an object are not brought to the same focus, and the object appears blurred. All eyes have this defect to a slight extent—to such a slight extent that it is not noticed. But let anyone draw a perpendicular line on a black board with chalk, let this line be crossed by another at right angles, and let the person, standing at a dis tance, look at the point where the two lines cut one another. Looking at this point steadily he will find that lie sees both lines, but one more distinctly than the other, showing both are not focussed at the same time. Of course one usually moves the eyes rapidly over objects looked at, and forms the idea of them from successive glances, without being aware that the whole object is not distinctly perceived at one glance. It is only when the defect is excessive that persons become aware of it. The error is corrected by the use of glasses ground from a cylinder ; ordinary glasses are ground from a sphere. Such spectacles are plane in one direction and concave or convex in the other. The glass is so placed that the direction of the curve is placed over the meridian of the eye requiring correction, and the degree of the curve is so arranged that by its means the cornea is made practically equally convex in all directions. Short-sight or long-sight is often associated with astigmatism, so that special spectacles require to be made which shall correct the long or short sight and the astigmatism as well. It is often a matter of very considerable difficulty to determine the degree of the defect, and the decision as to the sort of glasses required is often a very tedious process. Astigmatism is very commonly inherited.

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