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

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.

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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    muscle—they being simultaneously placed in a higher state of tension/' &c.* That the act of accommodation for near objects is not due to the action of the external muscles of the eyeball is, however, proved in an incontrovertible manner by a case of Yon Grasfe's, in which all the recti and obliqui muscles of both eyes were paralysed, so that the eyes were completely immovable, and yet the power of accommodation was perfect. It has at length, however, been definitely settled, chiefly by the experiments of Cramer and Helmholtz (conducted independently of each other), that the necessary change in the refraction of the eye during accommodation is due to an alteration in the form of the crystalline lens. Helmho]tz found, by means of Iris ophthalmometer, that the lens did not change its position during accommodation for near objects, but that this was brought about by a change in the curva- ture of the anterior and posterior surfaces of the lens, which become more convex (the lens itself thicker from before backwards), so that the lens acquires a higher power of refraction, and consequently a less focal distance, by which means rays from even very near objects are brought to a focus upon the retina. He found, with the ophthalmometer, that the eye undergoes the fol- io wing changes during accommodation for near objects : 1. The pupil diminishes in size. 2. The pupillary edge of the iris moves forwards. 3. The peripheral portion of iris moves backwards. 4. The anterior surface of the lens becomes more convex (arched), and its vertex moves forwards. * Aflt, vol. 3, 207.
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