Sight and touch : an attempt to disprove the received (or Berkeleian) theory of vision / by Thomas K. Abbott.
- Thomas Kingsmill Abbott
- Date:
- 1864
Licence: Public Domain Mark
Credit: Sight and touch : an attempt to disprove the received (or Berkeleian) theory of vision / by Thomas K. Abbott. Source: Wellcome Collection.
Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.
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![change in the convexity of the cornea, in the diameter of the eye-ball, or in the position of the lens. Young showed that there is no change in the cornea; and more recent and accurate observations have shown that there is a change in the form of the lens, the convexity of both its surfaces being increased in near vision, so that the anterior surface approaches nearer to the cornea, while the summit of the posterior surface re- mains (nearly) in its place. The method adopted in the in- vestigation was, to observe the images of a flame formed by reflection at the two surfaces of the lens and at the cornea respectively. It was found that the middle image diminished when a near object was looked at, indicating an increase in the curvature of the anterior surface. At the same time, it also approached the image formed by the cornea, in conse- quence of the advance of the summit of the lens.* These * The change in the form of the lens was first proved by Young, Philos. Trans., 1801; but his method was not very accurate. The ob- servations referred to in the text were first made by Cramer and Donders, in Holland, in 1851 [Cramer, uHet Accommodatie-Vermogen der Oogen, Harlem, 1853; in German by Doden]. See Schauenburg, uDas Accom- modations-vermogen der Augen nach Dr. A. Cramer und Prof. F. C. Donders, Lahr, 1854. Nearly at the same time Helmholtz independently made the same observations. Berlin Monatsberichte, 1853, p. 137. Graefe's Archiv fur Ophthalmologic, vol. i., pt. 2. Physiologische Optik, p. 110. See also J. D. Forbes, Edinb. Trans., vol. xvi. (1849), p. I. The history of opinions on the subject is given by Helmholtz, Physiol. Opt., p. 118. On the muscular mechanism by which the ad- justment is effected, see Henke, in Graefe's Archiv, vol. vi., pt. 2, also H. Miiller, ibid., vol. iii., pt. 1 ; Mannhardt, vol. iv., pt. 1; Czermak, vol. vii., pt. 1; ( Ueber das Accommodations-phosphen,) and Langen- beck, Annalesd'Oculistique, vol xxiv. The most probable hypothesis appears to be that in the position of rest the lens is flattened between the leaves of the zonula Zinnii, and in near adjustment the ciliary muscles or tensor chorioideae contracts, causing the zonula to relax and permitting the lens to assume its form of equilibrium, which is the more convex. If the tensor suddenly ceases to act, the tension of the zonula causes a tug on the retina; and hence Czermak accounts for the luminous ring which](https://iiif.wellcomecollection.org/image/b21037930_0104.jp2/full/800%2C/0/default.jpg)
