IT is sometimes disputed whether visual after-images are due to retinal or central processes. Their retinal origin appears to be capable of demonstration as follows. If one eye is pressed with the finger or a spring balance near the outer canthus for about thirty seconds, it will become temporarily blind. This effect, known to Thomas Young, Helmholtz and Donders, is attributed to retinal anoxæmia. The pressure should be about 250 gm., applied over an area of 1 sq. cm. The easiest method is to press on top of the lids, keeping them sufficiently far apart to permit vision. A brown mist will spread over the visual field, and finally all objects will disappear. The image of a bright light, such as a 60-watt bulb, at a distance of 1 m. or 2 m. should now be thrown on a fixed region of the retina in the blind eye.
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