26 



THE SIGNS OF LIFE 



[l.ECT. 



behaves itself with a more prolonged exposure of the eye. So 

 I adjust the shutter for such prolonged exposure to light 

 a minute will be a convenient time expecting to obtain, 

 if the eyeball is quite normal, a larger and increasing posi- 

 tive deflection during exposure. And now watch the spot 

 rather closely for what will happen when I cut off the light by 

 closing the shutter ; as you hear the click of closure, you see 

 that the spot makes a short positive excursion to your right 

 before falling back to its original starting position. All these 

 features will be best realised from a photographic record of the 

 travelling spot ; here is such a record, on which you readily 

 distinguish the large positive and increasing response to a 

 prolonged exposure, ending by the small positive response er- 

 as we familiarly call it in the laboratory the " parting bow " 



o mins. 



!0 



FlG. 10. Frog's eyeball. Galvanometric record of five successive 

 normal responses to light ; each illumination lasts for one minute. 



at the cessation of illumination, after which the current 

 gradually falls to its original level. This has been no excep- 

 tional result, but the typical and regular mode of response of a 

 normal carefully prepared eyeball ; the photographic record of 

 five such responses taken on a more slowly travelling plate will 

 be sufficient evidence of this. In each case there is deflection 

 in the positive direction at " make " of light, during light, and 

 at " break " of light. 



1 5. Further points. There are several subordinate points 



