The fingers when extended show a tremor of 8 to 12 movements per second. The problem was to describe the relation of voluntary movement to tremor. Three optical levers (Dorsey phonelescopes) were mounted so as to record photographically time, voluntary movement of a finger, and tremor. Finger depression proved either a continuation of the descending phase of a tremor or an interruption of an ascending phase. About 75% of elevations or depressions of either or both forefingers were of the first type, so the writer concludes that "a voluntary movement is, in most instances, a continuation of a tremor, and does not interrupt the tremor rhythm." Studies of three hemiplegics support the view that tremor is of cortical origin. "It appears that reaction time depends upon the temporal relation of the stimulus to the tremors and that the maximal rate of repeated voluntary movements of any given part of the body is limited by the tremor rate." (PsycINFO Database Record (c) 2006 APA, all rights reserved)