Page:Mind (Old Series) Volume 11.djvu/225

 224 J. M. CATTELL : being but little shorter than the actual time as determined by the tuning- fork. When we know the time between the closing of the current at the upper key, and the breaking at the lower, the strength f the current attracting the armature and of the spring holding it back can be so adjusted that the hands record the correct time. The stronger the current and spring are taken, the shorter is the time required for the armature to be attracted after the current has been closed and drawn back after the cur- rent has been broken. The determination with the tuning-fork need only be repeated so often that we are sure no error has been made ; it is well to change the distance between the keys and see that the times given by the chronoscope and the tuning-fork are the same. The chronoscope must, more- over, be controlled every day by the gravity-chronometer (or by a sensitive electrometer ; the apparatus itself is a very sensitive electrometer) to see that the current has remained constant, and to readjust it if it lias become stronger or weaker. For this purpose the gravity-apparatus supplied by Hipp can be used if proper precautions are taken. The strength of the current is adjusted by means of a rheostat, (B, ft, Fig. 8) and its direction changed (to avoid permanent magnetism) by means of a commutator. It is evident that a battery must be used giving as constant a current as possible. After considerable experiment I have adopted a form of the zinc-copper gravity-battery. I use six large cells, renewing them about once a month. If the chronoscope is properly controlled it measures the times very accurately. With the same current the mean variation of the chronoscope (including sources of error in the gravity-chronometer) is less than TOO sec. This small variation corrects itself completely in a series of measurements. A second variation about equal to the first is caused by the current not being accurately adjusted, or changing after it has been so adjusted. This error also tends to eliminate itself. A third source of error lies in the chronoscope's running too fast or too slow. This is, however, no greater than in any chronographic method where the time is measured by a vibrating tuning-fork ; the chronoscope can indeed be regulated with great accuracy as it runs a minute (60,000 vibrations). The gravity-chronometer (Fig. 1) was used in nearly all my experiments to produce the sense-stimulus, and to close at the same instant the current controlling the chronoscope. When the reaction-time for light was to be determined, the space between the columns was tilled up with Mack paste- board, so that the screen was completely hid from the observer. In the pasteboard (below the screen, the magnet being higher than in the figure) a hole 3x2 cm. was cut, and the observer fixated a black surface several mm. back of the hole. The experimenter allowed the white screen to fall by breaking the current which had been flowing through the coil of the magnet. Suddenly and without warning, at tlie point fixated by the observer a white >url'ace. 3x2 cm. appeared ; at the same instant (to sec.) the screen struck the lever of the key (Fig. 2) and rinsed the current controlling the chronoscope. N-> noi.-e i< made by the falling screen until it is stopped liy striking the spring /and the rubber cushions c r, and this noise comes too long alter the light to either shorten or lengthen the time of the reaction. The spring /is so adjusted as to partially Mop the falling si-iven and to prevent it from rebounding after it has struck the cushions. If cerebral operations other than those included in the reaction-time were to be investigated, the object exciting these operations, a printed word for example, was paMed on a card 15 x .'3 cm. This card is held in position by the springs <j </, and is hid from the observer by the black screen. The observer fixated a j^rey spot on the screen, which exactly covered the object on the card (the figure shows ot course the back of the apparatus). A bent