Eye disorders > Determining eye refraction

Determining eye refraction

In this section we will discuss the following subjective and objective niethods by which the condition of the refraction may be determined.

1. The acuteness of vision.
2. Scheiner's method.
3. The ophthalmoscope.
(a) The indirect method.
(b) The direct method.
(c) Retinoscopy.

In every case that presents itself we must proceed in a systematic manner, and before commencing to take the patient's visual acuteness, something may be gained by noticing the general appearance of the patient, the form of the face and head; thus a flatlooking face is sometimes an indication of hypermetropia ; a head elongated in its antero-posterior diameter, with a long face and prominent nose, may indicate myopia. If the two sides. of the face are not symmetrical, or if there he some lateral displacement
of the nose from the median line, astigmatism may be suspected. We should also notice the shape of the eyes themselves, if large and prominent, or small; in the former case we may suspect myopia, in the latter hypermetropia. In high degrees of astigmatism it can sometimes be seen that the curvature of one meridian exceeds that of the other. The distance between the eyes should also be noted, as well as the, direction of their-axes.

We next listen to the patient's own statement of the troubles from which he suffers; he may say that he sees distant objects well but has difficulty in reading, especially in the evenings, or that after reading for smne time the type becomes indistinct, so that he must
rest awhile, here we suspect hypermetropia ; or he may be able to read and do near work, but sees badly at a distance, then we suspect myopia; or both near and distant vision may be defective. In this. case our first object must be to decide whether the imperfect vision is due to some error of refraction or to some structural change. in the eyes themselves; and we possess an exceedingly simple, method by which to differentiate between them, and this method is called the Pin-hole test.

Pin-hole test - A black diaphragm having a small
perforation in its centre (the box of trial glasses usually contains such a diaphragm) is placed quite close to the eye uncler examination. This perforation gives passage to a small pencil of rays. which passes through the axis of the refracting system of the eye, so that the image formed is clearly defined for all distances: if then the pin-hole improve vision, the refractive system is at fault; but if, on the contrary, vision is not improved, then we suspect that the transparency of the media or that the retinal sensibility is defective ; thus we possess a very simple and
reliable plan, which if used systematically, may save much loss of time. The points to notice when applying this test are, that the illumination is good, and that the pin-hole is immediately in front of the centre of the pupil.

Having then found out that our patient's retrac=tion is defective, we proceed to the first method, the acuteness of vision.