44 



THE SIGNS OF LIFE 



[LECT. 



| 24. The effects of tetanisation upon the effects of light. 

 Experiment II. Our next experiment is to show that the 

 electrical response to light is modified by tetanising cur- 

 rents. I had to use very weak currents for the first ex- 

 periment, I shall use much stronger currents for this one, 

 because I want to show an unmistakable modification of the 

 electrical response to light, and I may use strong currents if 

 while they are passing they are short-circuited from the gal- 

 vanometer. First notice the response to light, it is about + 14 

 degrees of scale. Now, I plug out the galvanometer and tetanise 

 the eyeball for half a minute. I then unplug, and the spot has 

 flown off scale to your right. That has been what on first 

 witnessing it I called a blaze-current ; it has been provoked by 

 the tetanisation to which the eyeball has just been subjected. 

 But for the present it is not our principal concern. I bring the 

 spot back on to scale by means of a compensating current, 

 and as soon as the spot has come to comparative rest it has 

 been gradually falling off to the left by reason of the gradual 

 subsidence of the blaze-current we again take a reading of the 

 electrical response to light ; it is now 30 as compared with 14, 

 its value before tetanisation, i.e., the normal response to light 

 has been more than doubled in consequence of tetanisation. 



1] (5,000) 



O 5 iomins.0 |_ L 5 L \L L /5 L \_20mins. 



Before bebc3.nisa.bion. Afber bebc3.nisc3.bion. 



FlG. 20. Frog's eyeball. Influence of tetanisation upon the normal retinal 

 response to light. Ordinary arrangement of induction coil, fed by two Leclanche cells. 

 Secondary coil at 9 centims. (5000 units on Berne scale). After tetanisation the 

 positive response is considerably augmented, and falls during illumination. The 

 terminal positive deflection at break of light is almost completely abolished. 



This marked augmentation of response as to current may be 

 due to augmentation ^{voltage or of conductivity,? to both factors ; 



