On long, short, and weak sight : and their treatment, by the scientific use of spectacles / by J. Soelberg Wells.

  • Wells, J. Soelberg (John Soelberg), -1879
Date:
1862
Catalogue details

Licence: Public Domain Mark

Credit: On long, short, and weak sight : and their treatment, by the scientific use of spectacles / by J. Soelberg Wells. Source: Wellcome Collection.

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    The distance between the furthest and nearest point of distinct vision is called the territory or range of accommodation. The amount of this range varies, of course, according to the strength and efficiency of the ciliary muscle, the elasticity of the lens, &c. Bonders determines the range of accommodation thus :— I take the li])erty therefore of proposing a standard as a general expression for the range of accom- modation. The range of accommodation A (which may be calculated through the distance which p and r, the nearest and furthest extreme point of accommodation, are from the anterior surface of the crystalline lens), is given by the focal distance a of an ideal lens, which, placed upon the anterior surface of the crystalline lens, would afPord to rays emanating from the near point a direction as they came from the far point. We must suppose this lens to be a meniscus placed upon the an- terior surface of the crystaUine, because the accommo- dation depends almost exclusively upon a change in the convexity of the anterior surface of the lens.* We can .easily understand that a may be approximatively found by the formula :— 1—l=-andthatA=i p r a ^ Let us illustrate Donders' way of determining the range of accommodation by an example or two, first explaining, however, the following expressions : A means range of accommodation ; r, far point; p, nV point; oo, infinite distance ; \ foot; , inch ; line. * Gvfefe's Arcliiv., vol. iv. 1.
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