The eye in health and disease : with an account of the optometer, for the adaptation of glasses, for impaired, aged, or defective sight being the substance of lectures delivered at the Central London ophthalmic hospital / by Alfred Smee.

  • Smee, Alfred, 1818-1877.
Date:
1854
Catalogue details

Licence: Public Domain Mark

Credit: The eye in health and disease : with an account of the optometer, for the adaptation of glasses, for impaired, aged, or defective sight being the substance of lectures delivered at the Central London ophthalmic hospital / by Alfred Smee. Source: Wellcome Collection.

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    or curtain, placed, which not only excludes the most oblique rays, but also regulates the amount of light. By its power of cutting F'^a- 22. Fig, 23. oflf the most oblique rays spherical aberration is diminished. From its excluding rays of hght from the eye, an excess of rays is pre- vented from entering that organ and acting injuriously on the re- tina. Light reflected from any body will, after passing through a minute pin-hole, form an image of that body on any screen with- out the intervention of a lens (fig. 23). The iris also diminishes materially the angle of aperture for objects situated laterally, and hence contributes to prevent the necessity for adjustment to dis- tance where they are so placed. (43.) Our organs of vision enable us to judge whether an object be at rest or in motion. We appreciate motion from being obliged to move the eye to maintain a distinct image of the object; and in this case you also perceive that our sense of motion in the mus- cles gives rise to the idea of motion. The limited field of distinct vision must much contribute to the accuracy of otir idea of the motion of a body. You may observe many curious effects of apparent rest and motion on the river steamers, whether other boats are at rest or in motion arotmd you. To judge of motion in other bodies you must either be at rest or know your own rate of progression, or you may be led into curious errors. Motion directly towards or from you is a phenomenon very difficult to appreciate, but you will find that binocular vision assists you ma- terially to judge of the effect. Besides our estimate of motion by the muscles of the eyeball, we must also take into consideration the action of the muscles of the head in some cases: hence, our idea of motion as deduced from om sense of muscular movements is a very complex affair. (44.) The eye, in its normal state, is influenced by the seven