Modern optical instruments and their construction / by Henry Orford.

  • Orford, Henry
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Modern optical instruments and their construction / by Henry Orford. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    as far as the ciliary collar. Exactly in the centre of the retina is a yellowish, round, elevated spot, about ^ inch in diameter, having a minute indentation called the fovea centralis at its summit. This is the point of distinct vision, and the fovea centralis is the most sensitive part of the retina. About TV inch on the inner side of the yellow spot is the point at which the optic nerve spreads out its fibres to form the retina: this is the only spot on the retina which is not sensitive to light, and is therefore called the blind spot. The Crystalline Lens.—Within the eye, just behind the iris, is sus- pended a soft transparent body, called the crystalline lens, of the form of a doable convex lens, whose anterior surface is much less curved than the posterior. The crystalline lens is contained in a transparent capsule, and is kept in its place by the ciliary processes. It is composed of successive layers, Fig. 2, whose refract- ive indices increase towards the centre, its solid nucleus, A, refracting light most powerfully. It is easy to see that the action of the lens is more powerful than if it were composed of homogeneous substance having the same refractive index as the nucleus. For it may be regarded as the combination of a double convex lens c, Fig. 3, with two concave lenses, a and b. These concave lenses
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