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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    rest, in which the accommodation = 0. Let x be the distance of any object D from the eye; or, more accurately, the distance A D of the object d from the point A, which lies in front of the eye at a distance a b = b c = c the focal length of the eye itself. The distance b e of e, the crossing point of the rays of light, h from the cornea b, will be very nearly = c + —, in which the nearly constant coefScient Tc is determined by the refrac- tive indexes and the curvatures of the ocular media. The dis- tance c E of this crossing point E from c, the point of rest of h the retina, is = — Accurately to accommodate for d the eye X must undergo changes, which if the curvatures and thickness of its media remained unaltered would be expressed by an h elongation of its axis = —. In actual fact the accommoda- X tion depends upon a simultaneous change in the curvatures, the thickness, and the axial diameters of the refracting media; in which change the alteration of the curvatures of the crys- talline lens plays the chief part. For the better display of these changes we may reduce them all to an elongation of the axis; that is, when we speak of an elongation of the ocular axis, we mean thereby the sum of all the elementary changes of curvature, thickness, and dimensions, which, in re- spect of the position of the crossing point of the rays of light, have the same effect that would follow a simple elongation or simple shortening of the axis of the eye, if all its curvatures and thicknesses remained unchanged. Hence it is possible to k take the value — as the measure of the accommodation effort X required. Let y be the effort of accommodation, which is necessary to accommodate accurately for the distance x; so that w = —. X This formula will apply not only to all positive values of x, from a; = 0toa;=+oo, that is, for the distances of all