412 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



It i.s evident tliat the reactions on the weakest pressures are long, the sensation being near 

 the threshold. lu this case the reaction is always long, as the brain centers can not be lield in 

 a state of unstable equilibrium, lest the motor impulses be [1 u] prematurely discharged. The time 

 was for J 6.8 c, aud for 3 (T, shorter for a strong blow thau for a medium blow. The stroug blow 

 was just less than painful, but the intensities could not be measured, nor were the blows exactly 

 constant. The differences in time for D and A were fairly constant, being shorter for the jxiint 

 uearer the brain. In the two sets the differences were for J 8.8 a and 8.6 c, aud for C 1 ff aud 

 6.3 ff; but in view of our other experiments, and for reasons already given, we can not regard 

 these differences as the time of transmission in the median nerve. 



In order to keep the force of the blow coustant and to avoid a continual pressure on the 

 point of application we devised a secoud method, which can be recommended for authropmetric 

 and clinical work. AVe placed thin tin foil over the part of the limb to be stimulated, and this 

 was connected with one wire of the circuit. Then we allowed a hammer to fall on the tin foil. 

 The surface of the hammer which struck the foil was 10 mm. across, and was connected with the 

 other wire of the circuit. Consequently when the hammer struck the foil the blow was given aud 

 the circuit closed. The hammer swung in points aud fell from a height of 1.5 cm., the blow thus 

 being constant. The arm was held in position by being placed in a wooden case, and the blow 

 applied to the back or outside of the arm on points E and F, nearly opposite the ])oints B and D. 

 The back of the arm was used because it could be struck by the hammer when placed in an 

 unstrained position. 



Table X. — Iieaclioii-limes on hlows. J ami C observers. SOO reactions. 



In these experiments the reaction-time was shorter (for J 6.0 a, for C 1.5.1: a) when the blow 

 was applied nearer the wrist than when it was applied to the upper arm, exactly the opposite of 

 what was found on the op])osite side of the arm. The shorter reaction-iime with the lower point 

 was due to the fact that the blow was given over the bone, and was much stronger than when the 

 same blow was given in the muscle of the upper arm. 



In order to avoid the tin foil over the skin and the rebound (which would break the circuit) 

 we devised a hammer which worked in a very satisfactory manner. In its final form the instru- 

 ment is shown in fig. 7. 



The hammer swings in points, and is held up by the electro-magnet M. This magnet can be 

 placed at any height, and the velocity of the blow can thus be altered. In our experiments the 

 hammer fell a perpendicular distance of 20 cm. The area of the surface giving the blow can also 

 be altered, but in our experiments it was always circular, 10 mm. across, the edge beuig slightly 

 rounded. The arm of the hammer (1'5 cm. in length from the points to the center of the area with 

 which the blow wns given) is a very light aluminium tube, and the weight is almost exclusively 

 in the hammer at the point where the blow is given. The weight of the hammer can be adjusted 

 by means of weights which are screwed on the tip. In our experiments the weights (the hammer- 

 being in its points) were 15, 30, and 60 grams, and the force of blow was proportional to these 

 weights. Sixty grams falling 20 cm. gave a sensation which was just short of painful when the 

 blow was on a part of the body in which the skin is close to the bone.' 



The current was closed when the blow was given by means of the contact shown in the figure. 

 A small platinum spring makes the contact at almost exactly the instant the blow is given, and 

 the current remains closed until the larger spring is released. 



With this hammer we gave stimuli on various parts of the body aud measured the time of 

 reaction. It was necessary to place the part of the body in a horizontal position, aud this was 



' With this instrument experiments are now in progress in the psychological laboratory of Columbia College 

 ■which should yield interesting results. The accuracy of discrimin.atiou is measured with blows of varying force- 

 (Weber's law) and the correlation of velocity, mass aud area determined (e. g., is the sensation the same when thei 

 velocity is 1 and the mass 2 as when the mass is 1 and tlie velocity 2). 



