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6 70 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV
Put the 3 or 4 diopter lens in place in the metal holder, and the lantern slide of the radial lines (fig. 391) in the slide-carrier. Light the lamp and focus the slide by moving it toward or from the projection lens. Now introduce the 0.5 cylinder. Only the horizontal lines will be sharp with the axis vertical or only the vertical lines if the axis is horizontal. Put in front of the projection lens the black disc with an aperture of 7.5 mm. The image will be much improved. Remove this and put in place the disc with a pupil of 2.5 mm. If now the light is well centered the entire circle of the radial lines will be fairly good. The image will be rather dim, however.
Remove the small pupil and put in place the stenopaeic slit (fig. 3996) . Place the slit parallel with the axis of the cylinder and the lines will all appear sharp. This is because the slit allows the light to pass only along a line, thus eliminating most of the disturbing rays from the unequal curvature. People with astigmatism can partly overcome the trouble by narrowing the pupil and partly closing the eye-lids so that objects are seen through a slit something as in the experiment (§ 931 a).
§ 932. Anisometropia or unlike refraction in the two eyes.—
This is not a rare defect. One eye may be normal and one astigmatic, one with myopia and the other long sighted or normal, etc. Where the two eyes are different, the efforts to get a correct image are greatly hampered, for an accommodation which would give a correct image in one eye will make the image of the other eye more confused.
When the differences in the two eyes are considerable, the image of one eye is discarded, or the poor eye is turned aside (squinted) to get it out of the way, and one gets along with monocular vision.
To make this demonstration in the most perfect manner there should be two lanterns side by side, each projecting an image at the
§ 93 la. Preparation of the pupils and slit. — These are easily made by cutting out pieces of thin tin or other metal the size of the trial lenses and boring the holes and cutting the slit. Metal is recommended because the image of the crater must be focused on the pupil or slit, and paper or wood would be burned by the absorbed energy (§ 852.)