The determination of the necessity for wearing glasses / by D.B. St. John Roosa.

  • Roosa D. B. St. John (Daniel Bennett St. John), 1838-1908.
Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: The determination of the necessity for wearing glasses / by D.B. St. John Roosa. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    - 4» - strength, but if insufficiency exists, these dots will be separated laterally. If the insufficiency be of the interior, the image from the right eye will be on the left side, and vice versa. By placing other prisms base inward before the eye, the dots may be brought into the same vertical line. The strength of the prism re- quired will be the measure of the deviation. A candle may be used instead of a dot when the test is used for a distance beyond the one employed in reading. Pris- matic glasses, unlike convex or concave glasses, are seldom used for a long period of time, but usually only for a few months or a year or so at the most, as a means of toning up weak muscles. They have no such extended application as glasses for the correction of errors of refraction and accommodation. Prisms are sometimes used to give the eyes strength by gym- nastic exercises. If the desire be to examine the in- ternal rectus, the base is placed outwards. 'Phis displacing the rays towards its base and those from the object will be deviated outward, and the image will be formed upon the retina outside of the macula. The eye will endeavor to overcome this by rolling inward, which motion, of course, pulls the macula outward. It is very doubtful if this kind of exercise of the ocular muscles has a very curative value, although much was expected of it at one time. The dot test for insufficiency sometimes fails to reveal a considerable weakness of the in- terni, and the following one should always be
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