The photic orientation of Limax creeping geotropically upon a vertical plate is such that the phototropic vector determining the angular deflection ß from the vertical path is proportional to log I. This is proved by the fact that with horizontal illumination tan ß is directly proportional to log I; with non-horizontal light rays from a small source the ratio

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is directly proportional to log I (where A = the angle between light rays and the path of orientation), the vector diagram of the field of excitation being in this case not a right-angled triangle.

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