24 



THE SIGNS OF LIFE 



[LECT. 



has been nearly quite avoided by cutting the frog's head in half, 

 taking up the fragment with forceps, and trimming away the 

 orbit from the eyeball with scissors. The eyeball is now resting 

 upon the clay pad of an unpolarisable electrode, the pointed pad 

 of the second electrode touches the cornea, the actual contact 

 being by a droplet of salt solution. The electrodes are con- 

 nected to a galvanometer. The eyeball and electrodes are 

 enclosed in a black box with a hole, tube, and shutter opposite 

 the eye, so that when desired the latter can be exposed for any 

 required time to light of any required strength. 



The connections are such that the fundus of the eyeball is 

 attached to the south terminal, and the cornea to the north 

 terminal, of the galvanometer as in Fig. 3 or in Fig. 8, with the 

 keyboard unplugged. (The circuit includes a compensator and 

 a secondary coil, to be used in our next Lecture.) 



13. The initial cur- 



) ** 



rent. --Notice as a first 

 point that the natural cur- 

 rent of the eyeball has 

 given current through the 

 galvanometer from N to 

 S terminal, i.e., that within 

 the eyeball it has been 

 from fundus to cornea. 

 Notice further the fact that 

 this current is rapidly de- 

 clining. The spot deflec- 

 ted to your right is sink- 

 ing to your left towards 

 its zero point.* If we 

 could afford to wait, we should see the spot reach the zero and 

 continue its journey to the opposite end of the scale, indicating 

 to us that the eyeball current, at first directed from fundus to 



* The lecture- room is east and west, the former being the lecturer's end. 

 The galvanometer is on a bracket fixed to the east wall of the room, and the 

 movements of its suspended magnet are shown by a vertical spot of light 

 reflected to a transparent scale placed on the lecture-table. The audience 



FIG. 9. Normal declining current of a frog's 

 eyeball. The rate and shape of decline resemble 

 those of a blaze current. 



