Eye disorders > Determining eye refraction > Scheiner method

Scheiner method

Although this plan for detecting ametropia is now but little used, it is necessary the student should understand the principles upon which it is based. A diaphragm having two small perforations is placed in front of the eye we wish to examine; the perforations must be so near together that rays passing through them will enter the pupil (Fig. 36).

The patient is directed to look at a small flame 6 metres off ; rays emanate from this flame in all directions, some fall on the diaphragm, the greater number are thus cut off, but a few rays pass throiigh the two openings, and if the eye be adapted for the flame,
i.e. if it is emmetropic, these two sets of rays will meet exactly on the retina, forming there one image of the flame; if, however, the eye be hypermetropic (with suspended accommodation), then the two sets of rays will reach. the retina before meeting, each set forming an image of the flame (A, Fig. 36)

The greater the hypermetropia the further apart will the images be formed; these are projected outwards as crossed images, and the patient sees two images of the flame. That convex glass (from our
trial box) which, held behind the diaphragm, causes the flame to he seen singly, is a measure of the hypermetropia.

If the eye be myopic, then the two sets of rays will have crossed and are diverging when they reach the retina, where two images of the flame are therefore formed (c, Fig. 36). These images are
crossed again as they are projected outwards, and having twice crossed, homonpnous images are the result.

To find the amount of myopia we have only to find the concave glass which, behind the diaphragm, brings the, two images into one. To enable us to teal if the images are crossed or homonymous, it is usual to have in front of one of the perforations a piece of
coloured glass. We will suppose the diaphragm held so that the two openings are horizontal, that to the patient's right having in front of it a piece of red glass: if only one flame is seen the case is one of emmetropia ; if two images of it appear, one white, the other red, with the red to the left of the other, the images are
crossed, and the case is one of hypermetropia. If the red appear on the right, then the case is one of myopia. The further apart the images are, the greater is the ametropia.