340 Professor Oliver Lodge [June 1, 



solely by an intervening layer of badly conducting material, or of 

 conducting material with gaps in it ; but that when light falls upon 

 the retina these gaps become more or less conducting, and the nerves 

 are stimulated. I do not feel clear which part is taken by the rods 

 and cones, and which part by the pigment cells ; I must not try to 

 make the hypothesis too definite at present. 



If I had to make a demonstration model of the eye on these lines, 

 I should arrange a little battery to excite a frog's nerve-muscle 

 preparation through a circuit completed all except a layer of filings 

 or a single bad contact. Such an arrangement would respond to 

 Hertz waves. Or, if I wanted actual light to act, instead of grosser 

 waves, I would use a layer of selenium. 



But the bad contact and the Hertz waves are the most instructive, 

 because we do not at present really know what the selenium is doing, 

 any more than what the retina is doing. 



And observe that (to my surprise, I confess) the rough outline 

 of a theory of vision thus suggested is in accordance with some of 

 the principal views of the physiologist Hering. The sensation of 

 light is due to the electrical stimulus ; the sensation of black is due 

 to the mechanical or tapping back stimulus. Darkness is physio- 

 logically not the mere cessation of light. Both are positive sen- 

 sations, and both stimuli are necessary ; for until the filings are 

 tapped back vision is persistent. In the eye model the period of 

 mechanical tremor should be, say, -^ second, so as to give the right 

 amount of persistence of impression. 



No doubt in the eye the tapping back is done automatically by 

 the tissues, so that it is always ready for a new impression, until 

 fatigued. And by mounting an electric bell or other vibrator on 

 the same board as a tube of filings, it is possible to arrange so that 

 a feeble electric stimulus shall produce a feeble steady effect, a 

 stronger stimulus a stronger effect, and so on ; the tremor asserting 

 its predominance, and bringing the spot back whenever the electric 

 stimulus ceases. 



An electric bell thus close to the tube is, perhaps, not the best 

 vibrator ; clockwork might do better, because the bell contains in 

 itself a jerky current, which produces one effect, and a mechanical 

 vibration, which produces an opposite effect ; hence the spot of light 

 can hardly keep still. By lessening the vibration — say, by detaching 

 the bell from actual contact with the board, the electric jerks of the 

 intermittent current drive the spot violently up the scale ; mechanical 

 tremor brings it down again. 



You observe that the eye on this hypothesis is, in electrometer 

 language, heterostatic. The energy of vision is supplied by the 

 organism; the light only pulls a trigger. Whereas the organ of 

 hearing is idiostatic. I might draw further analogies between this 

 arrangement and the eye, e. g. about the effect of blows or disorder 

 causing irregular conduction, and stimulation, of the galvanometer in 

 the one instrument, of the brain cells in the other. 



