CHARLES E. DE M. SAJOUS, ASTIGMATISM.—Erom Gr., cc priv.; and ifTiyitcc. a point. This word, pro posed by Dr. Whewell, has been adopted, with slight modification, in all modern languages.
Astiginia, from a, and ariTP-/), is pro posed to replace it, as being etymologic ally better and shorter, although some of the derivative words would be longer. Georges Martin (Ann. d'Ocul., Mar., '95).
Definition. — That error of refraction by reason of which rays, coming from a single point and passing through the refractive surfaces of the eye, are not turned toward a single point, and, there fore, cannot be perfectly focused on the retina.
Irregular Astigmatism.
Definition.—The form of astigmatism arising when one or more of the refract ive surfaces of the eye is irregular; so that rays passing through different parts of these surfaces are turned in various directions and can never be brought to a perfect focus.
Symptoms.—There is imperfect vision, the blurring being proportioned to the degree of the defect and the size of the pupil, and affecting the seeing at all dis tances and all times. An eye subject to this defect is permanently "weak," can not attempt work requiring very accu rate seeing, and is liable to be strained in reading, sewing, etc. The irregularity of surface is generally accompanied by more or less haziness or opacity. This may be the opacity remaining in the cornea when irregular astigmatism has been caused by corneal inflammation, or it may be an opacity of the lens, when such astigmatism is the forerunner of cataract.
Pronounced irregular astigmatism causes monocular diplopia, or polyopia. A lamp-flame or the moon at night is seen multiplied, the different images of it usually overlapping each other more or less. It also shows itself by the dis tortion of letters, and in the appearance of additional lines about or upon them, plain type being made to appear like fancy type. The "rays" which appear to proceed from a point of bright light, as a star or a distant electric lamp, are due to irregular astigmatism. An eye free from astigmatism would see a star as a mere point of light.
Etiology. — Some irregular astigma tism is present in all normal eyes. When it causes no impairment of vision, below the usual standard of 20-xx, it is called normal. Normal irregular astigmatism
is generally caused by the inequality of curvature in the periphery of the dilated pupil, this being cut off when the pupil contracts.
The form of irregular astigmatism in which rays piercing the cornea in the same meridian, but at different distances from the centre, are differently refracted, while rays piercing it in different merid ians, but at the same distance from the centre, are equally refracted, is called the "symmetrical aberration" of the eye. It is "positive" if the rays are most re fracted at the edge of the pupil. This commonly depends upon increased curva ture of the periphery of the lens. It is "negative" when the rays are least re fracted at the edge of the pupil, from the flattening of the periphery of the cornea. Conical causes a form of high negative aberration. E. Jackson (Trans. Amer. Ophthal. Soc., 'SS).
Even when not excluded by the iris, such astigmatism may not cause imper fect vision, because the distinct retinal image may be formed from a small part of the light entering the eye; and, while additional unfocused light renders accu rate vision slightly more difficult and tiresome, it does not prevent it. E. Jack son (.Jour. of Amer. Med. Assoc., Sept. 1, '94).
Irregular astigmatism may be due to irregularity of the surface of the cornea, as from abrasion or superficial ulceration.
Irregularity of the corneal surfaces and of the layer of mucus covering the cor nea, which acts as a portion of the cornea in the refraction of light, may be caused by partial closure of the lids with press ure of the lid-margins upon the cornea, —that constriction of the lids which is designated in French dignement. The lid-margin resembles, somewhat, the rub ber scrapers used for cleaning windows, and constriction of the lids causes this prominent margin to make a groove on the cornea. Brief constriction merely displaces the viscid covering of the cor nea, and irregular astigmatism due to it disappears with a single sweep of the lids in winking. Prolonged constriction causes a groove in the corneal substance, the effects of which are rendered more evident by winking, and which is quite slowly effaced. George J. Bull (Trans. Eighth Inter. Oph. Cong., p. 107).