I thought it might prove of some interest to demonstrate at the psychological section the instrument I have hitherto used at the West Riding Asylum for the purpose of registering the reaction time to a sight and sound signal. The instrument in its original form, as made by the Cambridge Scientific Instrument Company, may be familiar to you, but lately I have so far modified it and extended its utility that I think it will be well to indicate the changes introduced in detail. The original, with minor alterations, has been described so fully in Dr. Hack Tuke's new “Dictionary of Psychological Medicine” that I shall only briefly refer here to the general scheme of its construction, referring those who seek more minute particulars to this article.