How to select spectacles in cases of long, short, and weak sight ... / by Charles Bell Taylor.

Date:
[1889]
Catalogue details

Licence: Public Domain Mark

Credit: How to select spectacles in cases of long, short, and weak sight ... / by Charles Bell Taylor. 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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    [4] pass a thread round this eyeball transversely in the middle, an imaginary plane perpendicular to the axis, dividing the globe into two halves,—that is, the equator,—and the segments respectively are the anterior and posterior hemi- spheres. Planes at right angles to the equator, vertical and horizontal, are called meridians, and the interspaces, for the purposes of description, are termed quadrants. Thus we say of the youth who has just come into the accident ward, that he has been wounded by the impact of a piece of metal which has penetrated the lower and outer quadrant of the left eye, that is, a space bounded above by the horizontal, and on the inner side by the vertical meridian. It is on the assumption that the eyeball is a sphere that it has been called the globe, and in truth in the human adult it is nearly a sphere, measuring almost, not quite, an inch in diameter, and weighing on an average a little over a drachm and a half. Slight variations in shape (normal astigmatism) are, however, universal, and I have known the eyeball so drawn out as to measure fully an inch and a quarter from pole to pole, and in another case so flattened as to present an antero-posterior diameter of only three-quarters of an inch. I have here another eyeball,—also, I perceive, a left one,— from which the sclerotic and choroid coats in the neighbour- hood of the optic nerve have been removed, in order that you may see from behind the images of the objects to which the cornea is directed focussed upon the retina. The same phe- nomena may be observed without dissection, if you can obtain the eye of an albino or of a white rabbit, in which the pigment of the choroid is absent and the sclerotic almost translucent, or, better still, on the focussing screen of the photographic artist who is about to take a picture. In all these cases you will observe that the image is inverted, and considerable ingenuity has been expended at various times in endeavours to explain how it is that we who go about with inverted images
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