THE TIME TAKEN UP BY CEEEBRAL OPEEATIONS. 221 two classes of processes, the one centripetal, the other centrifugal, there are centrimanent cerebral operations, some of which are given in consciousness, and make up the mental life of thought and feeling. These cerebral changes all take time, and, as I shall show, the times can in many cases be determined. I. Apparatus and Methods. The time taken up by cerebral operations cannot be directly measured. It is necessary to determine the time passing be- tween the production of an external stimulus, which excites cerebral operations, and the making of a motion after these ope- rations have taken place. The apparatus needed to determine this time must consist of three parts: (1) An instrument pro- ducing a sense-stimulus to excite cerebral operations and regis- tering the instant of its production ; (2) an instrument registering the instant a motion is made, after the cerebral operations have taken place ; and (3) an instrument measuring the time passing between these two events. The first two instruments must vary with the sense-stimulus to be produced and the motion to be registered ; to measure the times, I have used the Electric Chronoscope made by Hipp in Neuchatel. When properly con- trolled, this chronoscope measures the time as accurately as any of the chronographic methods which have been proposed, and it is much simpler and more convenient in its application. The Chronoscope is a clockwork moved by a weight and regulated by a vibrating spring. The spring vibrates a thousand times a second, and at each vibration the tooth of a wheel is allowed to pass, somewhat on the principle of the escapement in a watch. This method of regulating the clockwork is ingenious and accurate, but, especially in the new form of the chronoscope, is apt to get out of order. The value of the chronoscope con- sists in the application of an electromagnet. The hands recording the time are not in connexion with the clockwork, and consequently do not move when it is set in motion ; but, when an electric current is sent through the coil of the electromagnet, the armature is attracted, a system of levers throws the hands into connexion with the clockwork and they are set in motion ; and, again, when the current flowing through the coil is broken, a spring draws back the armature and the hands stand still. Thus the time the current flowed through the coil of the electromagnet is measured.i The hands record thousandths of a second. 2 The chronoscope works with great accuracy ; the only serious difficulty in its application being that the length of the times recorded by the hands varies with the strength of the current passing through the coil of the magnet. Supposing the strength of the spring holding back the armature to remain constant, if the current sent through the coil is very weak, the. soft iron is only completely magne- tised after a considerable interval, and it takes longer for the magnet to attract the armature after the current has been closed, than for the spring 1 A second electromagnet makes it possible to reverse this process, and measure the time a current has been broken. 2 Throughout this paper, both in the text and in the tables '001 second is taken as the unit of time. I use a- as a symbol to represent this unit : a- is analoous to n = '001 mm.
