Eyesight; And How to Care for It

Eyesight; And How to Care for It

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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1899 edition. Excerpt: ... cut into the usual oval form, and is set in the spectacle-frame with its axis corresponding to the direction of the correct curvature of the cornea. In this way we are able to place the proper convex or concave lens before one meridian of the cornea, and, at the same time, only a plain glass before the opposite meridian; or, if necessary, to correct each meridian separately by the appropriate curvature on each side of the glass. A difference of focus in the eyes of the same person is not uncommon. One eye may be correct, and the other long-sighted or short-sighted; they may have different degrees of the same defect, or one eye may be long-sighted and the other short-sighted. Sometimes, when the difference is very great, it is convenient to wear glasses that will adapt one eye to distant vision and the other to near. Irregular action of the external muscles of the eyeball is closely connected with the subject of this chapter, as it is usually dependent upon optical defects. When we look at a distant object, supposing the eyes to be optically correct, the axes of the eyes are parallel, and the muscle of accommodation is at rest--the image is formed upon the retina, in accordance with the physical laws of optics, without the necessity for muscular action, and vision is without effort. When, now, we look at some small object near at hand, both eyes must be directed towards it (their axes converged) by the action of the muscles that move the ball, at the same time that the focus is correspondingly altered by the action of the muscle of accommodation, increasing the convexity of the lens. Here, then, is a closely associated action of the muscles on the outside of the eye, and the muscle of accommodation within it, that adapts the focus to the...show more

Product details

  • Paperback | 34 pages
  • 189 x 246 x 2mm | 82g
  • Rarebooksclub.com
  • Miami Fl, United States
  • English
  • black & white illustrations
  • 1236663020
  • 9781236663023