Page:Mind (Old Series) Volume 11.djvu/231

230 J. M. CATTELL : varying most from the corrected average, which can usually be foreseen. In this paper I give the average of all the reactions made, as well as the average corrected by the method I have de- scribed. It will be seen that the two values do not differ greatly ; this is owing to the fact that the conditions of the experiments were such that really abnormal reactions seldom occurred. The second point to be mentioned here is the influence of practice, attention and fatigue on the length of the times deter- mined. In a later section of this paper I shall give an account of experiments I have made on this subject. In other cases it was sought to eliminate as far as possible these sources of variation. The two subjects (Dr. G. 0. Berger and the writer) on whom the determinations were made had already had much practice in psychological work. They were in good health and lived regularly, not even using coffee. The experiments were made every morning (except Sunday) from eight to one o'clock. After each series of 26 reactions, a considerable and constant interval elapsed before the same subject again reacted. The subject held his attention as constant as possible, and was not disturbed by noise or the presence of others in the room. These experiments, though begun in America, have been carried out in the psychological laboratory of the University of Leipsic. Professor Wundt, the founder and director of this laboratory, has earned the gratitude of all those interested in the scientific study of the mind. I owe him special thanks for the constant help and encouragement he has given me in my work. II. Tlie Reaction-Time. The reaction-time can be determined with ease and accuracy, but it is difficult to decide what operations take place when a re- action is made, quite impossible to determine how the time is divided among the several operations. We shall see that under favourable circumstances the reaction - time for light is about 150<r. It seems to me probable that this period is divided about equally between the processes occurring within and without the brain. The latter are : (1) the latent period in the sense-organ ; (2) the time of transmission in the afferent nerve ; (3) the time of transmission in the spinal cord and efferent nerve ; and (4) the latent period in the muscle. Physiologists have attempted to determine these times separately, but they must be far more constant than the discordant results would lead us to suppose. The experiments I am about to describe show that when the reaction-time is measured the mean variation of the separate times from the average is only ^j- of the whole time ; and we may attribute this small variation chiefly to changing 1 I call of the brain. If these times were not constant it is pro- person on wlioime could not distinguish colours and tones. v at which a nervous impulse is transmitted has