Page:Mind (Old Series) Volume 11.djvu/379

 378 J. M. CATTELL I When, however, lights of two different colours (say red and blue) are used, and the subject may only lift his hand if the light is blue, the motor impulse cannot be sent to the hand until the subject knows that the light is blue. The nervous impulse must therefore probably travel from the thalami to the cortex and excite changes there, causing in consciousness the sensation or perception of a blue light ; this gives a perception-time. In the cortex after the light has been distinguished a nervous impulse must be prepared and sent to the motor centre discharging a motor impulse there held in readiness ; this gives a will-time. I do not think it is possible to add a perception to the reaction without also adding a will-act. We can however change the nature of the perception without altering the will-time, and thus investigate with consider- able thoroughness the length of the perception-time. The object most quickly perceived through the sense of sight is a simple light. In order to investigate the time required I took two cards, one entirely black, the other having on the black a w r hite surface. One of the cards, the observer not knowing which, was placed by the experimenter in the springs of the gravity-chrono- meter, and the clockwork of the chronoscope was set in motion. The observer fixated the grey spot on the screen immediately before the centre of the white surface (supposing this card to be there), and with his left hand broke an electric current and let the screen fall. The card appeared at the point fixated, and at this same instant the current controlling the chronoscope was closed. The observer either saw nothing, or at the point fixated a white surface. If the light appeared he lifted his hand as quickly as possible, if there was no light he did not let go the key, and the hands of the chronoscope ran on until the clockwork was stopped by the experimenter. Twenty-six experiments were made in a series, the white light occurring thirteen times. Deter- minations were only made when the light occurred, so the averages in this section are from thirteen reactions (in the corrected series from ten). It will be seen that, as the observer tries to make the reaction as quickly as possible, he may lift his hand when the light is not present. If this happens often the times measured are not correct, but too short, since we may assume that the observer lifts his hand as often when the white light is ] >r. -sent before he has seen it, as he makes the motion when no light comes. We must however expect such a false reaction occasion- ally to occur, otherwise we might assume that the reaction is not made in the minimum time when the light is present. In these experiments such false reactions scarcely happened except when the observer was disturbed, or when the impressions to be dis- tinguished were similar (E from F, for example). In the first case the average is not seriously affected, as the reactions are as apt to be unduly retarded as unduly hurried. In the second case false reactions lead us to suppose that some of the reactions on the stimulus are too short. The method I have introduced of giving