100 THE SIGNS OF LIFE [LECT. 



The nerve-skin response, now before your eyes, is a very 

 good case to use for familiarising you with some of our 

 apparatus. 



We guessed the lost time at three seconds, we will now make 

 a rather more accurate measurement by photographing a galvano- 

 metric deflection, and finally we will control our measurement 

 by taking an electrometer photograph. ( Vide infra, p. 105.) 



Two galvanometers are in circuit in series (as described 

 in the Appendix, p. 1 56) ; the recording galvanometer or galvano- 

 graph will take on the sensitive plate a replica of the indica- 

 tion witnessed by you on the demonstrating galvanometer, and 

 it will be rather interesting to you, perhaps, to see whether 

 the impression on your mind is borne out by the impression 

 on the photographic plate. The plate is set to fall at the 

 rate of about 2.5 mm. per second (or an inch in 10 seconds), 

 and the instant of stimulation is signalled on the plate by 

 a device that you can examine afterwards. I start the clock- 

 work, and 2 or 3 seconds after you have heard the commence- 

 ment of excitation (of the nerve), you see the deflection 

 caused by the electrical change that has taken place in the 

 skin. When the plate has got to the end i.e., after 40 seconds 

 it is shut up in its carrier and sent to the developing-room, 

 from which it will be brought back in a few minutes, and 

 placed in the projecting lantern (Fig. 43). 



Meanwhile let us examine the response by means of another 

 instrument the capillary electrometer put into the circuit 

 instead of the galvanometers (Appendix, p. 162). The 

 magnified image (x about 1500 diameters) of the mercury 

 column, projected on the transparent screen, looks to you like 

 a large manometer and, in point of fact, it is an electrical 

 manometer, as you see at once if I raise or lower the elec- 

 trical pressure in circuit by, e.g., thousandths of a volt. 

 Having verified that the connections are such that movement 

 of the mercury upwards on the screen signifies outgoing 

 current, and downwards the reverse, we may proceed to excite 

 the sciatic nerve as before and watch the electrometer image 

 on the screen. It reacts perfectly well by a downward move- 

 ment each time I excite the sciatic nerve and my impulse 



