The theory of ocular defects and of spectacles / translated from the German of Hermann Scheffler by Robert Brudenell Carter with prefatory notes and a chapter of practical instructions.

  • Scheffler, Hermann, 1820-1903.
Date:
1869
Catalogue details

Licence: Public Domain Mark

Credit: The theory of ocular defects and of spectacles / translated from the German of Hermann Scheffler by Robert Brudenell Carter with prefatory notes and a chapter of practical instructions. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    (les rciimlichen Sehens, I have given these limits for normal eyes, and for lenses of various power. For example, Fig. 9. normal eyes with orthoscopic convex spectacles of 100 inches focal length, would not see clearly at a greater distance than 100 inches; with spectacles of 7-inch focal length, at a greater distance than seven inches. And just as convex spectacles curtail the farthest distance of the range of vision to the limit of their own focal length, so they also cirrtail the middle distance, or the range of clearest vision. Convex spectacles of 7-inch focal length would, for example, curtail the middle distance to four inches : that is to say, a pair of normal eyes, in order to get the clearest possible image through such spectacles, would require the object to be as near as four inches to them. The concave orthoscopic spectacles have the contrary effect; and, for normal eyes, they increase the distance of the middle visual region, and leave distant vision wholly unlimited. Con- cave spectacles limit the region of vision, therefore, towards the near point. Concave spectacles of 100 inches focal length remove the point of clearest vision to a distance of ten inches;