Determination of the refraction of the eye by means of the ophthalmoscope / by Edward G. Loring.

  • Loring, Edward G. (Edward Greeley), 1837-1888.
Date:
1876
Catalogue details

Licence: Public Domain Mark

Credit: Determination of the refraction of the eye by means of the ophthalmoscope / by Edward G. Loring. 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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    As the very word refraction implies the true optical value of an eye independent of its accommodation, it follows that this condi- tion can only be ascertained when the eye examined is in a state of rest. Further, that it is indispensable that the observer should be aware of the exact state of refraction and accommodation of his own eye, before he can estimate that of another. Perfect relaxation of the accommodation in the observed eye can of course be obtained by atropia, no matter what the nature of the refraction is. But usually, sufficient relaxation can be se- cured in emmetropia by causing the patient to look into the dis- tance, and as much as possible into vacancy, which is induced somewhat by having the walls of the ophthalmoscopic room painted black. For a myope it w411 only be necessary that he should look at some object which is at a greater distance than his far point. The ability and disability which hypermetropes have in relaxing their accommodation will better be considered a little later under its special heading. As far as the observer is concerned, it can be laid down as a rule, at least for beginners, that the nearer the refraction of his eye approaches emmetropia, and the more completely he can relax his accommodation, the better. This ability to relax the accommodation varies with different people, some accpiiring the power completely, others only partially. Practice here, as else- where, increases the ability. If the observer is emmetropic, one of the best methods of acquiring this control over the accommo- dation is to take a convex glass of a moderate j^ower, say -g-, and ascertain the farthest point at which fine type can be read with perfect distim^tness through the glass, the other eye being closed, or better still, opened but excluded from the visual act by a screen. Under this condition there is a tendency for the visual axes to assume a parallel position, and with it that perfect state of rest usual to the eye when looking at the most distant objects. If the object can be moved in this case to a distance of eight inches, it is proof positive that the accommodation is entirely relaxed, since, as the object viewed is situated at the principal focus of the glass, only parallel rays can enter the eye, and such rays can only be brought to a focus on the retina of an emmetropic eye when it is in a state of perfect rest. This experiment should be repeated 2 •