54 THE SIGNS OF LIFE [LECT. 



" On the blaze-currents of the Eyeball." That the electrical 

 response of the eyeball to light has its exclusive seat in the 

 retina, is proved by the fact that of the two halves of an eyeball 

 bisected into an anterior and a posterior half, only the latter 

 responds to light, and if further as recommended by Kiihne, 

 the retina itself be separated from the remaining coats, electrical 

 response to light persists of the isolated retina, but is absent 

 from the remaining tissues. But a similar procedure in the case 

 of electrical response to electrical excitation taught me that the 

 reaction is not confined to the retina, that it is manifested by 

 the anterior as well as by the posterior half of the eyeball, that 

 the isolated cornea and the isolated lens give blaze-currents. I 

 therefore concluded that tissues other than retinal are coeffective 

 in the electrical response to electrical stimulation of the entire 

 eyeball, and accepted the fact as a hint to examine other living 

 tissues for this presumably common sign of life. Meanwhile, 

 from the retinal sheet in animals excitable by light, my attention 

 naturally turned to the chlorophyl sheet in vegetables presum- 

 ably excitable by light, and a demonstration of the electrical 

 response to light in green leaves was the immediate result* I 

 tried petals of flowers in the same way, as one among other 

 control tests of the reality of the leaf response. Petals proved 

 to be absolutely inert in this respect. Yet petals are obviously 

 " alive," so I tested petals by the blaze-test, and found that to 

 this test petals are at once found to be alive. The idea by this 

 time had fully generalised itself in my mind, and I placed all 

 sorts of living things, animal and vegetable, on the unpolarisable 

 electrodes, leaves, stems, seeds, fruits, etc., muscle, nerve, lung, 

 liver, pancreas, skin, hen's eggs, etc., and found that some things 

 reacted well, and others not at all, or irregularly. But these 

 were digressions digressions, however, that have in my opinion 

 become far more interesting than the original questions. 



34. To return to the second of these. To get an indica- 

 tion whether the same or two different substances react to 

 light and to induction shocks, I looked for modifications of the 



* WALLER. " The Electrical Effect of Light upon Green Leaves," 

 . Roy. Soc., vol. 67, p. 129, 1900, 



