The harmonius symmetrical action of the oblique muscles in all cases of oblique astigmatism : read before the Nashville Academy of Medicine, January 15, 1891 / by G. C. Savage.

G. C. Savage

Date:: [1891]

Credit: The harmonius symmetrical action of the oblique muscles in all cases of oblique astigmatism : read before the Nashville Academy of Medicine, January 15, 1891 / by G. C. Savage. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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$hovizontal the less oblique are the images. What is true of hypermetropic astigmatism is also true of the myopic and mixed forms of astigmatism. The obliquity of the image varies not only with the difference in po,>ition of the meridian of greatest curvature, but also with varying quantities of astigcnatism. The image will not bo so oblique, if there is 1 D of astigmatism, with its best meridian at 135° (or 45°), as when there are 2 D,' in the same direction. The amount of obliquity of the retinal image of any horizontal object varies, in any given quantity of astigmatism, widi the more or less remote position of ^the meridian of greatest curvature from the vertical or horizontal up to 45°; also with the quantity of astigmatism, the best meri- dian being at some fixed angle between the vertical and horizon- tal. In small degrees of astigmatism the obliquity of the image can be but a few minutes, while in the higher degrees the obliquity is 5° or more. In some of the figs, used illustrating this paper the obliquity of the image is represented as being as much as 10°, only for convenience of illustration. All errors of refraction should be corrected by the examina- tion of only one eye at a time, an opaque disc being placed in front of the other eye, and the eyes should be under the influence of a mydriatic, unless ciliary strain has been suspended by age ; especially should this be the plan of procedure in all cases of oblique astigmatism. Tt is in cases of oblique astigmatism only that oculists find trouble in locating the axis of the cylinder, at one moment the patient indicating that it should be at a certain angle, and at the next moment 5° or more removed from this point. Enough time should be spent and sufficient care exercised to enable the operator to determine the natural location of the best meridian, and at this angle the axis of the cylinder should always be placed. If the best meridian in hypermetropic astigmatism is in the temporal quadrant and there is doubt as to whether the axis of the cylinder should be placed at a certain angle or 5° nearer the horizontal the latter should be chosen, for in such a case the superior oblique from long habit wants to continue to revolve the eye in an abnormal, but, without the proper cylinder, necessary, position. The point to be chosen.$