Prism exercises : their indications and technique / by Alexander Duane.
- Alexander Duane
- Date:
- 1904
Licence: In copyright
Credit: Prism exercises : their indications and technique / by Alexander Duane. 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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![A progressive increase in the effect may be produced not only | in the way above indicated, i. e., by using different combinations j of two or three prisms, but also by using a single prism and ' rotating it on a vertical axis either forward or back. The de- flecting power of a prism can be almost doubled in this way, and, moreover, can be increased gradually, so that with two or three prisms thus rotated all degrees of deflection may be produced from 5° to 50° of arc. If with any given combination the diplopia cannot at once be overcome, resort may be had to Gould’s expedient of approximat- ing the test-object until seen single, and then gradually carrying it off to the proper distance, the patient all the time keeping his eyes on it and endeavoring to maintain fusion. Or, the same thing may be effected if, as suggested by Stevens, the patient looks through the prism at a finger held close to his nose, and keeps his eyes fixed sharply on the finger as it is slowly carried out toward the distant object. Exercise B. Exercise zmth prisms, base out, at near points. — Exercise at near points with prisms, base out, is done in the same way as for distance, except that the test-object is either a minute electric light or a fine dot’^ in the center of a circular card five inches in diameter. The card is held at the reading distance, and j is shifted back and forth, special efforts being made to maintain ! fusion while the card is being carried toward the eyes. | Exercise C. Exercise at near points zmth prisms, base in. — ] As is well known, the ability to overcome prisms, base in, when | the eyes are directed at a distant object, cannot usually be in- I creased by practice. This does not hold, however, for such prisms when used at near points. The test-object here is the same as that used in Exercise B, i. e., is either a minute electric light or a dot on a card. The pa- tient, holding a 12° or 15° prism before either eye, brings the object up toward him until it appears single, then carries it steadily off until he can no longer fuse the double images. This is done three or four times in succession, the attempt always being * If there is hyperphoria, the dot may conveniently be replaced by a short vertical ine.](https://iiif.wellcomecollection.org/image/b22447374_0008.jp2/full/800%2C/0/default.jpg)
