Hints on spectacles : when required and how to select them / by W. Ackland.

  • Ackland, William.
Date:
[1867]
Catalogue details

Licence: Public Domain Mark

Credit: Hints on spectacles : when required and how to select them / by W. Ackland. 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.

    8/40 (page 8)
    We clearly know it exists, we appreciate its advantages; but whilst we may boast of tracing so far the refined contrivances of this admirable organ, we need be under no false shame in pointing out something that eludes our scrutiny. One set of philosophers assert that the power is pro- duced by the simultaneous contraction of all the straight muscles of the eye, producing pressure on the orbit, compressing the fluids within, and causing its elougation. Another opinion is, that when we wish to examine near objects, the crystalline lens is increased in curvature, and certainly the structure of that lens, and the surround- ing ciliary process, lead me to lean to this mode of expla- nation. It matters little whether we lean to the one or the other theory, so that in practice we give due weight to this power, when we come to speak of supplying the deficiency of vision by spectacles. Long Sight*—We have already stated that the healthy eye can distinguish near and distant objects with equal facility. By this we mean any object not nearer to the eye than eight inches; but at a certain period of life long-sightedness occurs, which shows itself by a difficulty of distinguishing any object closer to the eye than fourteen, sixteen, or eighteen inches, although objects at a distance are as easily seen as in youth. When this occurs, the rays of light from a near object, o, after being refracted by the eye, fail to form a sharp image, on the retina, but behind it at i, as on Fig. 4 ; hence, instead of accurate vision, imperfect haze is the result, and clearness can only be obtained by removing the