Explanatory text to Wood's ophthalmic test-types and color-blindness tests / by G. R. Cutter.

  • Cutter, George R. (George Rogers), 1840-
Date:
1880
Catalogue details

Licence: Public Domain Mark

Credit: Explanatory text to Wood's ophthalmic test-types and color-blindness tests / by G. R. Cutter. 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.

    8/16 (page 8)
    In simple astigmatism the state of the refraction of the one principal meridian is emmetropic, and that of the other is either myopic or hyper- metropic. Simple astigmatism is divided into: 1, simple myopic astigma- tism, in which myopia exists in the one principal meridian, and emmetro- pia in the other. It is noted as Am; 3, simple hypermetropic astigma- tism, in which there is hypermetropia in the one principal meridian and emmetropia in the other. It is noted as Ah. In compound astigmatism myopia or hypermetropia exists in both prin- cipal meridians, but the degree varies. There are also two forms of this variety: 1, compound myopic astigmatism (noted as M-j-Am), the de- gree of myopia varying in the two principal meridians; 2, compound hypermetropic astigmatism (noted as H-|-Ah), the degree of hyperme- tropia varying in the two principal meridians. Mixed astigmatism, in which the one principal meridian is myopic and the other hypermetropic, is a rare form. Two varieties have been dis- tinguished: 1, mixed astigmatism with predominant myopia (noted as Amh); 2, mixed astigmatism with predominant hypermetropia (noted as Ahm). Astigmatism is of comparatively rare occurrence, and requires for its correction the aid of cylindrical lenses. The adjustment of these lenses is a matter of considerable intricacy, and should be undertaken only by those who have made it a special study. Our trial case, though it does not contain cylindrical lenses, furnishes the means for the detection of as- tigmatism, and an approximate estimation of its degree. In astigmatism, as in the other cases, each eye should be examined separately. The direction of the group of lines wliich appears most dis- tinct should be ascertained, and the small card with three parallel lines placed with the lines at the angle, noted. Then place the stenopaic slit in the frame in front of the eye, but at an angle perpendicular to that of the lines being tested. Then find the convex or concave glass which renders the lines most distinct. When found note the angle and the number of the glass. The lines of the small card are then to be turned 90 degrees, the stenopaic slit turned to correspond, that is at right angles to the lines, and the glass found which renders the lines distinct in their new position. When found, note the angle and the glass as before. The difference between the two glasses represents the degree of astigmatism. In making up the combination for the correction of the astigmatism, it is customary to give the stronger refraction to the spherical glass, and the weaker to the cylindrical lens. The accuracy of the combination may be tried on either of the cards which contain lines. If all the lines running in different directions appear equally distinct, the astigmatism is corrected for the distance. Before this is accomplished, however, it may be found necessarv to make slight changes in the combinations. In noting the combination put the letter s after the spherical glass, then the sign of combination and the letter c after the cylindrical glass, with the degree of its axis, reckoned from the horizontal meridian from the left toward the right, thus: -f 3 D. s. O — 1 D c, axis 90°. After correcting the astigmatism for the distance, the near vision should be corrected with the aid of the finer lines.