in.] TETANISATION 43 



pectation is realised by the following experiment, in which, 

 as you will readily see from the diagram, a weak tetanising 

 current is to be passed through a secondary coil, an eyeball, 

 and a galvanometer, all the plugs being removed. The 

 tetanising current is so weak that its constituent single shocks, 

 alternating in direction, do not of themselves affect the gal- 

 vanometer, but do stimulate the eyeball, which gives current 

 in the circuit, current which, as you see from the deflec- 

 tion of the spot to your right, is in the expected (positive) 

 direction. 



I repeat the experiment, having turned over a reverser in 

 the secondary (or it might have been in the primary) circuit, 

 in order to reverse the directions of the constituent shocks, 

 and the spot is again deflected to your right. I take it that 

 this experiment needs no comment ; it says that induction 

 currents of both pairs of directions arouse a positive electrical 

 response of the eyeball. 



In ordinary language, the "effect" of a modifying cause is 

 subsequent to that cause, and is therefore, properly speaking, an 

 " after-effect." But it is in some instances necessary to distinguish 

 between the effect during its cause, and the after-effect subse- 

 quent to that cause. You have just witnessed the effect during 

 tetanisation necessarily during weak tetanisation. I cannot 

 show you an effect during strong tetanisation, since strong 

 induction currents traversing the galvanometer would mask the 

 physiological current of the eyeball. If you should wish to 

 observe the effects of strong tetanisation, you must be content 

 with what are properly speaking after-effects, since I must put 

 the galvanometer out of circuit during tetanisation, and unplug 

 it to receive the eyeball current that may be present as the effect 

 or after-effect of strong tetanisation. And I will do this now, to 

 show that the effect (or after-effect) of tetanisation may be very 

 great indeed. The eye current is exactly compensated so that 

 I can plug and unplug the galvanometer without disturbing the 

 spot. I plug the galvanometer and tetanise the eyeball. Then 

 I unplug and the spot flies off scale to your right, indicating 

 positive current in the eyeball as an effect (or after-effect) of the 

 tetanisation. 



