Image-changes caused by astigmatism and by correcting cylinders / by G.C. Savage.

  • Savage, G. C. (Giles Christopher), 1854-1930.
Date:
1897
Catalogue details

Licence: Public Domain Mark

Credit: Image-changes caused by astigmatism and by correcting cylinders / by G.C. Savage. 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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    object is looked at. In fusing any other parts, the complicated func- tion must be performed. Only a correction of these cases of astigma- tism by proper cylinders can give to the recti muscles the privilege of performing only their simple function. Astigmatism with the merid- ian of greatest curvature horizontal, in one eye, and no astigmatism in the other is worse than if both eyes were astigmatic with meridians of greatest curvature horizontal, for the reason that, in the latter, images in the two eyes being similar the recti muscles are called on to do only work that is normal; while in the former, images being dissimilar from side to side, the lateral recti muscles must do abnormal work (perform their complicated function). When their is no astigmatism in one eye and astigmatism with meridian of greatest curvature vertical in the other, the condition is worse than if both eyes were astigmatic with eridians of greatest curvature vertical, for the reason that in them latter, images being similar, the recti muscles must do only normal work; while in the former the vertically acting muscles must do abnor- mal work (perform their complicated function). The worst kind of vertical astigmatism exists when the meridian of greatest curvature is vertical in one eye and horizontal in the other, for then images are un- like both vertically and horizontally, and therefore require of all the recti muscles the performance of the complicated function, except when vision is fixed on the center of the object looked at. In binocular astigmatism, the meridians of greatest curvature be- ing parallel, whether vertical or oblique, images must be similar. This being true the extra-ocular muscles have to perform only their simple function—do such work as they must do in emmetropic eyes. The correction of such cases of astigmatism is important that the ciliary muscle may be relieved from abnormal work. In all other kinds of astigmatism correcting cylinders are needed to relieve not only the ciliary muscle but also one or more pairs of the extrinsic ocular muscles. I have proved in this and in former papers that there is distortion of retinal images in oblique astigmatism; and that the distortion is in opposite directions in the two eyes when the meridians of greatest cur- vature diverge or converge above. This much is already generally conceded even by my former critics. To state that such images can be wholly fused would not be correct; but that fusion is attempted by the harmonious symmetric action of the oblique muscles cannot be de- nied without, at the same time, denying the existence of the supreme law of binocular single vision, the law of corresponding retinal points. The most daring would hardly deny the latter. Figure 3 shows how a square would be seen as a non-rectangular parallelogram (a'-b-d-d)