The errors of accommodation and refraction of the eye and their treatment : a handbook for students / by Ernest Clarke.

  • Clarke, Ernest, 1857-1932.
Date:
1911
Catalogue details

Licence: In copyright

Credit: The errors of accommodation and refraction of the eye and their treatment : a handbook for students / by Ernest Clarke. Source: Wellcome Collection.

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

    184/250 (page 168)
    The abducting power of the normal extemi is equal to a prism of 7° or 8°. Insufficiency of the extemi is generally associated with hyperopia. We have seen that in hyperopia, accommodation is required in excess of convergence, and, unless the two can be dissociated, the hyperope must converge to a nearer point than is necessary. This over-contraction of the interni causes latent convergence for distance, and also for the near point (Fig. 72, D), the external recti become insufficient, and extra impulses must pass down to these muscles in order to obtain fusion vision. This may lead to fatigue and eyestrain, which, how- ever, often disappear, owing to the development of a convergent squint and loss of binocular vision (see page 86). Treatment.—By putting the hyperope into glasses we re-establish the harmony between convergence and accommodation, remove the spasm of the internal recti, and consequently also the insufficiency of the extemi, restoring the balance of all the muscles. One of the pleasantest things to do in ophthalmic work is to cure a squint, or a tendency to squint, by simply giving the patient glasses. If parents would only realize that, in a large number of cases, this can be done by bringing the child early enough—i.e., before the latent squint has become manifest, or possibly during the early period of the manifest squint—there would be fewer squints. It is very seldom that we have to resort to prisms in the treatment of esophoria, but, should it be found necessary, the prism must be placed base out, and if decentring is substituted, convex glasses must be decentred outwards (see Fig. 74). Note.—Method of finding the Amount of Decentring necessary to produce the Efpct of a given Prism in a given Lens (Ward Hold en). —^Take 8.7 mm. as the distance a lens of i d must be moved, to produce the effect of a prism 1°, as the unit, multiply 8.7 mm.