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CH. XV] DEMONSTRATING REFRACTIVE EYE DEFECTS 671
same time. If, however, direct current is available the demonstration is successful with one projector. There are needed :
(1) A trial frame with half the lens of 4 diopters and half of 3 diopters (fig. 400).
(2) A trial frame with half the lens of i diopter and the other half of plane glass.
(3) A lantern slide of fig. 401.
For the demonstration the arc lamp and the lens should be so related that the image of the source of light is rather large as shown
Left
Eye Eye
FIG. 401. LANTERN SLIDE FOR THE FIG. 400. DOUBLE TRIAL DEMONSTRATION OF UNLIKE REFRAC
LENSES FOR UNLIKE RE TI°* '* THE Two EYES
FRACTION IN THE TWO EYES.
by the concentric circles in fig. 400. The light must be accurately centered also.
Put the lens in the metal holder and the special lantern slide in its carrier and move the slide up to a point 27 to 28 centimeters from the lens. The image of the right eye (4 diopter lens) will be sharp, and that of the left eye will be blurred. Now pull the slide back to a distance of 36-37 centimeters from the lens and the left eye image will be sharp and the right eye blurred. This is comparable to a defect of myopia in one eye and hyperopia in the other — one eye is short sighted and one long sighted. Put the slide back in position for the 4 diopter lens so that the right eye will be in focus. Now put in front of the lens the correcting lens of i diopter for the left half. This will make both sides of the lens 4 diopters and both images will be sharp as in normal vision (fig. 402, A.B.).
It will be seen that the blurred, left-eye image (fig. 402 A) is smaller than the sharp right-eye image. This is because the 3