Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D.

  • Jackson, Edward, 1856-1942.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D. Source: Wellcome Collection.

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

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    the astigmatism. Such a change in the direction of the axis of the cylinder is to be made, and the test repeated until the correction of any remaining astigmatism conforms exactly with the direction of the lens before the eye. This remaining astigmatism must be corrected by a change in the strength of the lenses employed. For example: Suppose an eye to have compound hyperopic astigmatism corrected by 4- 4. sph. 3 + 2- cyl- axis 900. The first inspection of the pupil shows the light moving against the light on the face in all meridians. Con- vex lenses 2. D. and 4. D. placed before the eye show the same thing. Convex 6. D. shows the light moving against the light on the face from side to side, but with it in a verti- cal direction. It thus becomes evident that astigmatism is present. Still stronger convex lenses are to be tried. The 8. D. lens shows movement in the pupil with the light on the face in all meridians. The 7. D. lens shows movement very indefinite or indistinguishable in the horizontal merid- ian, but clearly with the light on the face in the vertical meridian. This lens then brings the point of reversal for the less myopic [more hyperopic without the lens] merid- ian to the surgeon's eye. The next step is to bring the original source of light closer to the mirror, so as to cause the immediate source of light to fall at the point of reversal for the more myopic [less hyperopic] meridian, which will now be one-third of a metre from the patient's eye. To do this [supposing that the mirror has a focal distance of one-quarter of a metre, ten inches] it will be necessary to bring the source of light to within two-fifths of a metre of the mirror. That is, the immediate source of light to be at one-third of a metre from the patient, must be at two-thirds of a metre from the mirror corresponding with 1.5 D. of the focusing power. The total focusing power of the mirror being equal to 4. D., the light must be so placed that the divergence of its rays