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Sine-wave rotary and rectilinear oscillation provided the stimuli for the study of the sensitivity of the several vestibular organs. Voluntary, manual, compensatory movement of the arm and hand and, in the case of rotary oscillation, of the fingers was employed as means of response to the perception of oscillatory motion. The apparatus for rotary oscillation consisted of a rotating platform, a driving mechanism and a recording system. The rotating platform was the same as that used by Dodge in his previous studies of rotation. A rectilinear platform which was connected with the rotating platform and driving mechanism of the latter was used for rectilinear oscillation, and the platform was driven by the same Pathé-motor driving-rod mechanism. A recording system similar to that for rotary oscillation was employed. Two subjects (the writers) were used. Various predetermined frequencies and amplitudes were employed in the case of both rotary and rectilinear oscillation. No