Skiascopy and its practical application to the study of refraction / by Edward Jackson, A.M.,M. D.

  • Edward Jackson
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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    principles underlying the test, it must remain a blind routine and rule of thumb work, and can never be of the highest utility. To aid in such an understanding of them, one may, in connection with the study of the succeeding chapters, take a strong (15 D. to 20 D.) convex lens and a piece of card-board with a dot on it. The lens can repre- sent the dioptric media of the eye, the card-board the retina, and the dot the light area upon the retina. The card-board should be held back of the lens, a little farther than its focal distance, and the dot looked at through the lens from various distances. Nearer the lens an erect image of the dot (blurred of course), and, farther away, an inverted image will be seen, and between the two the phenomena of rever- sal. The movement of light on the retina may be imitated by a slight movement of the card in different directions. The apparent enlargement of the dot, as the point of reversal is approached, and the diminution of its apparent size as the point of reversal is departed from, its diffusion and indistinctness near the point of reversal, and its con- centration and greater definiteness away from the point of reversal, are to be observed. Such a combination of dot and lens will also beautifully exhibit the phenomena of aberra- tion [See Chap. V] with its central and peripheral areas of differing movement, the one an erect and the other an invert- ed image. The difficulty of keeping the dot in view when the point of reversal is approached, will illustrate how small a portion of the retina is visible from the point of reversal when the test is applied to the eye. By holding in combination with the spherical lens a cylindrical lens of 5 D., the distortions of the light area produced by astigmatism, and the band-like appearances it causes at certain distances, should also be studied. This is not all to be done at a single lesson, but the lens and card should be kept at hand where they can be
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