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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    which would receive a circle of diffusion if the rays were coming from the point B. Or, again, if the observer's eye be placed at C, from which rays will be focused at c in front of the retina, and, at which, rays coming from c would be focused, he will be able to perceive the portion of the re- tina, m n included, between the dotted lines, passing through c and continued on to the retina—the area upon which would be formed a circle of diffusion by rays coming from the point C. It follows then that the closer the observer's eye to the point of reversal, the more is the real movement of light upon the retina magnified, and, therefore, the swifter does it appear. Tixe farther the observer's eye is removed from the point of reversal, the less is that real movement of light on the retina magnified; and the slower is the apparent movement as watched in the pupil. And, as this source of variation overcomes all other sources of variation in the rate of the apparent movement of the light [except the rate of rotation of the mirror, which is, to a considerable extent, under the control of the ob- server] , the rapidity of the apparent movement of light and shade in the pupil increases as the point of reversal is approached, and diminishes as that point is departed from, and constitutes a measure of the degree of ametropia remaining uncorrected. Form of the Light Area.—The real form of the light area on the retina, except under certain conditions in astig- matic eyes, will be circular. If the light be perfectly focused on the retina it is circular, because that is the form of the source of light employed (see Chapter III). If the light be not perfectly focused on the retina, the circular pupil gives its form to the resulting area of diffusion. The influence of regular astigmatism on the apparent form of the light area as seen in the pupil will be discussed in Chapter IV ; and the influences of irregular astigmatism and aberration in Chapter V. These influences, especially the latter, are really dominant, and of the greatest practical