On the Artificial Dissociation of the Relationship between Accommodation and Convergence
If someone holds a weak prism before the eye with its vertical base outward he sees crossed double images at first, but they are soon single again because he eliminates the double images by an increase in the convergence of the eyes. If he uses a strong prism for the same experiment, when he is not practiced in the observation of peripheral double images, he does not see double but does see various things unclearly through one another. He sees a different part of visual space with each eye and is not able to reestablish clear vision unless he finds the necessary eye position by chance. The observer who is practiced in such experiments indeed immediately notices the corresponding double image for each image, if it is at all recognizable, and easily reestablishes single vision by strong convergence. In this way I myself have overcome every prism which does not demand a stronger inward turning of the eye than the anchoring of the eyeball permits.
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