Refraction of the eye : its diagnosis and the correction of its errors / by A. Stanford Morton.

  • Morton, A. Stanford.
Date:
1886
Catalogue details

Licence: Public Domain Mark

Credit: Refraction of the eye : its diagnosis and the correction of its errors / by A. Stanford Morton. Source: Wellcome Collection.

    47/88 page 35
    The further the departure from this degree, which we may call i D. {■^) of M., the larger and feebler, as was seen on the screen, will be the illuminated portion of the retina. cL^'yr^^^'^^'in^J^^ The difference in the degree of luminosity of this reflex can be easily appreciated, and furnishes a means whereby the amount of error maj be approximately estimated : so that, speaking generally, we may say, the feebler the illu- mination the higher is the ametropia. An attempt has been made to use, for the same purpose, the difference in intensity of the shadow, but the appreciation of this is difficult, and it therefore does not afford an altogether re- liable source of information. If, from some distance, say i m. 20 cm., we examine an eye with the mirror alone, we find that the higher the H. or M., the smaller is the image which we obtain of the disc: so that in very high degrees, we see not only the whole disc, but also some of the surrounding fundus in the pupillary area. In E., on the other hand, so large is this image that only a small portion of it is visible at one time. With equally rapid rotations of the mirror then, the light would have to travel much faster over the large image of the latter, than the small image of the former, condition. This difference in the ra^e of movement in the various states of refraction, was pointed out by my friend Dr. Charnley : it constitutes, especially when taken together with the limmosity of the image, probably the best of the means at our disposal for estimating, by this method without glasses different degrees of ametropia. In E. and the lower degrees of H. and M., so little is
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