On the Artificial Upward or Downward Deviation of One Eye
I have referred above to one more fact which appears to speak against the hypothesis of always equal innervation of both eyes. If we hold a very weak prism with the base up or down in front of one eye, a previously fixated point appears in double images lying over one another, though they flow together after a while. The double images fuse because the eye behind the prism is rotated slightly upward or downward, so that both lines of sight no longer lie in one plane. It is best to arrange the experiment so that the weak prism is first held with the base outward and then, after simple vision is reachieved, is slowly rotated about the line of sight until the base lies exactly upward or downward (Donders). The more often one performs this experiment, the stronger the prism which one can use in this way. I have myself used a prism mounted in an eye-glass frame and have gone to continuously stronger prisms and finally compensated for one of eight degrees, though only with great effort and for a very short time.
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