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.

    105/122 (page 99)
    THE MIRROR. The size of the mirror will depend somewhat upon the purpose for which skiascopy is to be used. If the mirror is to be employed to measure refraction of all kinds, to show the movement of light in the pupil with high uncorrected hyperopia or myopia, it must be large, to give the range of movement for the immediate source of light that is neces- sary to render evident the direction of movement in the pupil, when that movement is slow and the illumination of the area is comparatively feeble. The disadvantage of a large mirror is that it gives a large area of light on the face, especially when as with the plane mirror, the original source of light is brought close to it. And in this large area of light on the face only the light reflected by a small portion of the mirror immediately surrounding the sight hole is of any use when the point of reversal is near to the surgeon's eye [see page 29 for dis- cussion of limits of the part of the retina visible in the pupil]. With a small mirror, making a small area of light on the face, it is easier to keep this upon the eye than it is to keep the similar limited portion of a large area properly directed. On this account, where skiascopy is used, after an ap- proximate estimate of the refraction has been made by the ophthalmoscope or other means, quite a small mirror is found convenient. By a large mirror is meant one from 35 to 50 mm. in diameter. By a small mirror is meant one under 20 mm. in diameter. The mirror, or, at least, the opaque back that carries it, cannot be well reduced to less than 20 or 25 mm., because, if smaller than this, it will admit light to the eye from the original source, through the space around the mirror; and such light, though not so annoying as a reflection at the sight hole, is a serious hin- derance in the application of the test. The mirror plate then must be large enough to shade the eye. A large mirror having a metal cap with an aperture of