The prescribing of spectacles / by Archibald Stanley Percival.

  • Percival, A. S. (Archibald Stanley), 1862-
Date:
1910
Catalogue details

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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    MEASUREMENT OF A 1. The most usual plan is to let the patient hold in his hand the small test type, and to note the shortest distance at which he can distinctly see the type with each eye separately. It will be found that he can only maintain his greatest accommodation for a few moments, so that the test need only occupy a minute or so. If the patient be myopic, or if the amplitude be very great, this method is unreliable, as the difference of W in. will introduce an error of 1 D in the determination of A, when the near point is only 4 in. off. For instance, in an emmetrope where P = 4 ins. or metre, A would be 10 D ; for in emmetropia R = go , . \ ^ = 0 so A or p - i = 10 - 0 = 10 D ; but if P were 3yTr ins. or TV metre, A would be 11 D. In all cases in which P is small, it is advisable to give concave glasses for the patient to wear, and to repeat the test. Suppose in the above case when —3D were worn P were found to be \ metre (5f in.), A = 7D - (- 3D) = 10 D. In fact in every case A = p — G — ^ where P and R are measured in metres, and where G denotes the power of the lens worn by the patient during the test. A difficulty will also arise if the patient be very hyper- metropic, as in that case the distance of the punctum proximam may be inconveniently great ; it will even be negative when only converging rays come to a focus on the retina. Thus, suppose a hypermetrope of 8 D with A = 5 D. In such a case R = — -§■ metre, and P = — J metre. The amplitude (A) could only be found by providing the patient with convex glasses. Suppose that + 5 D are given him ; on now testing his near point, P will be found