A manual of the principles and practice of ophthalmic medicine and surgery / by T. Wharton Jones.

  • Jones, Thomas Wharton, 1808-1891
Date:
1847
Catalogue details

Licence: Public Domain Mark

Credit: A manual of the principles and practice of ophthalmic medicine and surgery / by T. Wharton Jones. Source: Wellcome Collection.

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    great as to forbid extraction, but of course, if, in the cases in which the vitreous body is so much dissolved, that the displaced lens is apt to float up again, displacement be con- tra-indicated, extraction is much more so. 1495. All other things being equal, it might perhaps he laid down as a general proposition, that in the very cases in which displacement admits of being most readily and safely performed, extraction is less safe, whilst, on the other hand, in the cases in. which, in consequence of the soundness of the vitreous body, extraction is most safely and easily performed, displacement is least so. 1496. As the cases for which division is best fitted are different from those in which extraction or displacement is indicated, there is no comparison to be made between them. It is, however, to be observed, that a combination of division and extraction is sometimes had recourse to in cases of common lenticular cataract of old people. The object of having recourse to this compound operation is, as above mentioned (s. 1456.), that the lens may, by solution and ab- sorption of its soft exterior part, be reduced to its hard nucleus, which, in consequence of its small size, will admit of being extracted through a small section of the cornea. Cataract glasses. 1497. The difference in refractive power between the air and the cornea, being much greater than between the aqueous humour and crystalline body, the greatest amount of refraction which the rays of light undergo in the eye, in order that they may converge to foci on the retina, is that effected by the cornea on their first entrance. The crystalline body contributes comparatively little to the con- vergency. Hence, vision, after a successful operation for cataract, may be still tolerably distinct for objects at a cer- tain distance. Still, in order, that it may be perfectly dis- tinct, the use of convex glasses is required. 1498. But as with the loss of tbe crystalline body, there is loss of the power of the eye to accommodate itself to dif- ferent distances, except so far as variations in the size of the pupil contribute to tliis effect, glasses of different de- grees of convexity are required according as the patient wishes to view near or distant objects. Thus, convex lenses of two and a half inches focus are generally required for