Physiologic optics : dioptrics of the eye, functions of the retina, ocular movements and binocular vision / by M. Tscherning ; authorized translation from the original French edition., specially revised and enlarged by the author by Carl Weiland.

  • Marius Tscherning
Date:
1904
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Licence: In copyright

Credit: Physiologic optics : dioptrics of the eye, functions of the retina, ocular movements and binocular vision / by M. Tscherning ; authorized translation from the original French edition., specially revised and enlarged by the author by Carl Weiland. Source: Wellcome Collection.

Provider: This material has been provided by The University of Glasgow Library. The original may be consulted at The University of Glasgow Library.

    98/392 (page 70)
    15.12 mm. The position of the real center would be 13.78 mm. (i) and the radius of the real surface 13.78 mm. — 3.73 = 10.05 mm. 34. Direct Determination of the Radii. — In fig. 49, as well as in figs. 50 and 51, the ratio between the distances separating the two images of the same kind is equal to the ratio between the apparent radii. We may, indeed, consider the distance separating the two lamps as an object, three images of which are formed on the pupil; these images are pro- portional to the radii following the formula = ~, since O and / are the same in the three cases. We can make sufficiently accurate measurements of the radii if we make use of two cursors similar to A and two others similar to B. We place the lamps A in such a position as to be able to observe clearly the images produced by the anterior surface of the crystalline lens. Then we displace the cursors B, the lamps of which must be feeble, until the corneal images of the lamps of each are on the sam.e straight line as one of the crystalHne images of A. We consider the distance which separates the cursors A as object for the anterior surface of the crystalline lens, and that separating the cursors B as object for the cornea. As the images are alike, the radii must be inversely propor- tional to the objects. Knowing the radius of the cornea, we can, there- fore, calculate the apparent radius of the anterior surface of the crys- talline. To determine the astigmatism of the surface we must repeat all the measurements in the vertical meridian. The posterior surface of the crystalline lens is measured exactly like the anterior surface. As to the posterior surface of the cornea, its image is not visible at the middle of the pupil. We must, therefore, limit ourselves to measuring the peripheral parts. The direct determination of the position of the surface, following the method indicated in para- graph 32, is not applicable for the same reason, but the position of the center can be determined after paragraph 33, and the length of the radius as we have just explained, which gives indirectly the thickness of the cornea. It is necessary to have previously measured the radius of the anterior surface of the cornea at the place where we are making the measurement, for generally this place is so peripheral that the flattening of the cornea makes itself felt. Besides, the posterior sur- (1) [Considering that we have again obtained this apparent position with reference to the refraction of the cornea, we must therefore in the forranla + - 1 put F, =- 23.64; Fj = 31.61 and fi 18.28, this gives A = 13.78.]-ir.