MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



399 



The current in these wheels does not pass through the points of the axis (which might cause 

 variation in the resistance), but through a platinum spring, <S', which touches the side of the wheel. 



The falling screen and contacts described make a cliroiioscopc useful for many psychological, 

 physiological, and perhaps physical purposes. We have used the iustrument chiefly to sinuil- 

 taneously close and break currents, and to give a standard 

 interval of 100 ff (one-tenth second) for the regulation of the 

 chronoscope. In two other directions, however, experiments 

 have been begun with the instrument. Electric shocks are 

 produced separated by (say) 10 a. These shocks are applied 

 to the nerve at varying distances from the brain, the eailier 

 shock having farther to travel. When the shocks are felt 

 simultaneously we have measured the time it takes to travel 

 the distance between tlie two contacts. This method involves 

 several psychological difticulties, but we hope to secure results 

 supplementary to those given in this paper. In the second 

 place the instrument has been used to uncover any object and 

 register the instant at which this occurs. Thus a printed 

 word may be covered by the screen and the screen be allowed y,„ ^ 



to fall. The instant the word becomes visible a current is 



closed and the time recorded. Thus the time required to see the word, for the association of ideas, 

 etc., may be measured. The special experiments now in progress consist of uncovering two 



Fig. 5. 



sirrfaces nearly alike in intensity and measuring the time required to perceive which is the 

 brighter. The time becomes longer as the difterence is taken less, and one mental quantity is 

 thus determined as the function of anotlier. 



