Publisher's Synopsis
This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1882 edition. Excerpt: ... chapter X. refractive disorders. There are several ways of determining the refraction of an eye. It may be found by means of the incident light; by means of the lens necessary to use in order to see the upright image; by means of the enlargement of this image; by the distance at which the inverted image is formed; and by the enlargement of this image. The way most used, however, because the simplest and easiest, is that of finding the lens required by the examiner, (any ametropia of his being first corrected, and the accommodation of both examiner and patient being fully relaxed, ) to clearly and distinctly see the fund us of the eye under examination. Myopia.--It will be remembered that in myopia, the rays converge to a focus before reaching the retina. In other words the principal focus of the dioptric media lies in front of the retina. Hence rays issuing from the retina of a myopic eye do not emerge from the cornea parallel, as is the case in emmetropia; but coming from a point beyond the principal focus, they emerge as convergent rays. They cannot as convergent rays be brought to a focus upon the observer's retina, but after they cross each other and become divergent, the observer can obtain a distinct image. The image is an inverted one, as is proven by the fact that it moves in the opposite direction from the observer's head. The head of the observer must be from twelve to sixteen inches from the eye under examination. An erect image may be seen at a short distance by interposing a concave lens behind the mirror; but the lens should be of sufficient power to render the convergent rays parallel. The focal length of this lens will approximately represent the degree of the myopia existing in the eye under observation, and therefore the..