Manual of instructions for the guidance of army surgeons in testing the range and quality of vision of recruits, and in distinguishing the causes of defective vision in soldiers / by T. Longmore.

Longmore, Sir Thomas, 1816-1895.

Date:: [1864]

Credit: Manual of instructions for the guidance of army surgeons in testing the range and quality of vision of recruits, and in distinguishing the causes of defective vision in soldiers / by T. Longmore. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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$In consequence of this defect when the images of objects are clearly formed on the retina in one of these directions, the images formed on the retina in the other direction across it are blurred and indistinct. It may depend upon corresponding differences in the amount of curvature of the horizonal and vertical meridians of the cornea, which has been shown to be its usual cause, [cylindrical eye,] or to an abnormal position or unequal curvature of the crystalline lens. The difference in the distance at which objects arranged linearly can be distinctly seen in a vertical direction as compared with the distance required for distinct vision in a horizontal direction, in- dicates the degree of astigmatism. Correction.—Astigmatism is corrected by lenses so formed as to act on one meridian of the eye only,—astigmatic lenses. They may either increase the refractive power in the direction of the one meridian, or lessen it in that of the other; by either means the focal power can be equalized in both directions. Such lenses must represent by their form portions of cylin- ders; the rays striking them in one direction will be then refracted as by an ordinary lens of corresponding curve, in the other direction they will pass through with no other refraction than that due to the medium through which they are trans- mitted. Astigmatism having been found to be a much more common source of defective vision than has been hitherto supposed, attention should be directed to the question of this disability existing in cases when the diagnosis is not otherwise clear and satisfactory. The vertical and horizontaljlnes in Snellen’s test- types offer a ready means of diagnosis for this defect. It can also be detected by placing between the eye and the sky or a brightened sheet of paper, a card with a small hole made by a pin in its centre. On slowly moving the card from the cylindrical eye, the hole will not appear to remain circular, but at a certain distance will change into a straight line, and at a still further distance this line will become changed in position, and assume a direction at right angles with its first direction. Astigmatism may be complicated with a generally myopic or hypermetropic condition of the eye.$