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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    er's eye is withdrawn from the observed eye, and approaches the point of reversal for the more myopic meridian, the light area in the pupil becomes elongated in this meridian ; and, while the movement is still with the light on the face in all meridians, it becomes more rapid in the direction of this elongation than in the direction perpendicular thereto. The observer withdrawing his eye still farther, on reaching the point of reversal for the more myopic merid- ian, [ V, in figure 9,] is unable to distinguish the movement in this meridian, while the movement in the meridian at right angles to it is still with that of the light on the face. This point being reached, if the original source of light be pushed away from the mirror, so that its reflection, the immediate source of light approaches the point of reversal for the less myopic meridian, the form of the light in the pupil becomes a distinct band running in the direction of the more myopic meridian, readily seen to move from side to side, but without perceptible movement in the direction of its length. Bringing the source of light back to its usual position close to the mirror, and withdrawing his eye still farther from the eye under observation, the observer again sees the movement of the light in the pupil in all directions. But in the direction of the most myopic meridian, it is now against the light on the face ; while in the meridian at right angles to this, it is still with the light on the face. The band-like appearance is now lost entirely; the area of light in the pupil taking at one distance the same shape as though no regular astigmatism were present. But, as the point of reversal for the less myopic merid- ian is approached, elongation in the direction of that me- ridian may be noticed, and the movement of the light in that meridian with the light on the face becomes more rapid than the movement against the light on the face now seen in the more myopic meridian. When the point of reversal