The prescribing of spectacles / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1910
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Licence: In copyright

Credit: The prescribing of spectacles / by Archibald Stanley Percival. 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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    CORNEAL IMAGES way with prisms, by finding the strength of prism that will abolish this apparent movement. This test I personally consider unreliable, for in the first place it requires an intelligent patient for its perception, and as the movement of redress is in the opposite direction to that of the deviation, it is sometimes this movement of redress which is noticed, and not the primary jump, so that confusion may easily result. I may say that Dr. Duane has himself pointed out these sources of error. The screen test is sometimes used with the object at a distance of J metre or so, but I prefer to use Harman's diaphragm test for near vision. 2. Corneal Images. Objective.—Mr. Priestley Smith and Dr. Maddox are the ablest advocates of this exceed- ingly useful method of detecting slight deviations ; the reader is referred to Dr. Maddox's book on The Ocular Muscles for a fuller account of the procedure. The patient is directed to look at the central hole in the mirror of the ophthalmoscope, with which the surgeon, being about a foot distant, throws the light first on one cornea and then on the other. The corneal reflection of the illuminated mirror appears as a small bright spot of light which maps out with sufficient precision for clinical purposes, the point in each cornea which is traversed by the visual line. It is important (a) To keep the patient's attention fixed on the mirror, as then the visual line of the patient coincides with that of the surgeon ; (b) To judge of the position of the bright spot with reference to the cornea and not to the pupil, which is usually eccentric. In normal eyes these corneal reflections occupy symmetrical positions in the two corneas, nearly always slightly to the inner side, for a reason that will be given later. If there is asymmetry, heterotropia is present. In this way the distinction between real and apparent squint can be at once made.