28 



THE SIGNS OF LIFE 



[LECT. 



zero current, i.e., whether the accidental current upon which it 

 is superposed be positive or negative. Or to use a familiar 

 laboratory mnemonic the normal excited eyeball always " looks 

 through its own galvanometer." (See Fig. 3). 



There is always a considerable interval of time between the 

 incidence of light and the electrical response ; sometimes the 

 delay at " make " of light may be so great as to be measurable 

 by a stop-watch, but in such cases the delay at " break " is much 

 less considerable, and I have considered the interval at make 

 as a period of hesitation (vide infra) rather than as a true 

 physiological lost-time. But even under the most favourable 

 conditions, with fresh and typically reacting eyeballs, I have 



FlG. 12. Frog's eyeball. Electrometric record of the normal response at 

 beginning (a) and end (u>) of illumination. The retinal delay is in each case about 

 sec., i.e., approximately the same as that of cortical grey matter. 



never seen the latency as short as given by Dewar and 

 MacKendrick, and more recently by Fuchs viz., " less than 

 T^th of a second." The shortest intervals I have measured 

 have been of about i second, no difference being detectable 

 between the make and the break deflections in this respect. 

 These values have been obtained from galvanometer records, of 

 which Fig. 1 1 is an example, and are subject to a correction, by 



