Skiascopy and its practical application to the study of refraction / by Edward Jackson.

  • Edward Jackson
Date:
1898
Catalogue details

Licence: Public Domain Mark

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

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    FORM OF THE LIGHT AREA. importance in nearly all eyes. But in eyes free from such defects the form varies with the departure of the observer's eye from the point of reversal. If the magnification of the Fig. 5. Fig. 6. retina is so slight that all of it occupied by the light area is visible in the pupil at one time, that area appears circu- lar, as represented in figure 5. But when the point of re- versal is approached so that the magnification of the retina prevents all of the retinal light area from being seen at one time, only a portion of its outline is visible as an arc of the greatly enlarged circle, as shown in figure 6 ; and the nearer to the point of reversal that the observer comes, the nearer does the boundary between light and shade approach to a straight line. It must be borne in mind, however, that this is still part of the boundary of a circle, and hence that dif- ferent parts will run in all the different directions, in con- tradistinction to the band-like appearance of astigmatism, the direction of which always conforms to one or the other of the principal meridians. From the pointy of reversal, however, but a single point of the retinal light area could be visible to the ob- server at a time, so that the form of that area could not from this position influence the form of light and shade apparent in the patient's pupil. From this single luminous point of the patient's retina the light is not focused on the observer's retina, but falls there in an area of diffusion which wouldjake its form from thejpupn the observer, or
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