June 7. 1894] 



NATURE 



I in— connected to a single voltaic cell ; a film of what may be 



lied oxide intervenes between the surfaces, so that only an 



,>if;nificant current is allowed to piss, because a volt or two is 

 insufficient to break down the insulating film except perhaps at 

 one or two atoms. If the film is not permitted to conduct at 

 all, it is not very sensitive ; the most sensitive condition is 

 stained when an infinitesimal current passes, strong enough 

 i ist to ^how on a moderate galvanometer. 



Now let the slifjhtest surging occur, say by reason of a sphere 

 1 ling charged and discharged at a distance of forty yards, the 

 film at once breaks down — perhaps not completely, that is a 

 question of intensity — but permanently. .\s I imaginr, more 

 molecules get within each other's range, incipient cohesion sets in, 

 and the monient.nry electric quiver acts as it were as a flux. It 

 is a singular variety of electric welding. A stronger stimulus 

 enables more molecules to hold on, the process is surprisingly 

 metrical : and as far as I roughly know at present, the change of 

 resistance is proportional to the energy of the electric radiation 

 from a source of given frequency. 



It is to he specially noted that the battery current is not 

 iit-eded to effect the cohesion, only to demonstrate i:. The 

 battery can be applied after the spark has occurred, and the 

 re-istance will be found changed as much as if the battery had 

 been on all the time. 



The incipient cohesion electrically caused can be mechanically 

 destroyed. Sound vibrations, or any other feeble mechanical 

 disturbances, such as scratches or taps, are well adapted to 

 restore the contact to its original high-resi-tance sensitive con- 

 dition. The more feeble the electrical disturbance the slighter is 

 the corresponding mechanical stimulus needed for restoration. 

 When working with the radiating sphere at a distance of 

 forty yards out of window, I could inot for this reason .sliout 

 to my assistant, in order to cause hjm to press the key of the 

 cod and make a spark, but I showed him a duster instead, this 

 being a silent signal which had no disturbing effect on the coherer 

 or tube of filings. I mention forty yards, because that was one 

 of the first outdoor experiments ; but I should think that some- 

 thing more like half a mile was nearer the limit of sensitiveness. 

 However, this is a rash statement not at present verified. At 

 forty yards the exciting spark could be distinctly heard, and it 

 was interesting to watch the spot of light begin its long ex- 

 cursion and actually travel a distance of two or three inches 

 before the sound arrived. This experiment proved definitely 

 enough that the efficient cause travelled quicker than sound, and 

 disposed completely of any sceptical doubts as to the sound-waves 

 being perhaps the real cause of the phenomenon 



Invariably, when the receiver is in good condition, sound or 

 other mechanical disturbance acts one way, viz. in the direction of 

 increasing resistance, while electrical radiation or jerks act the 

 other way, decreasing it. While getting ih^ receiver into condition, 

 or when it is getting out of order, vibrations and sometimes 

 electric discharges act irregularly, and an occasional goo i 

 shaking does the filings good. 



I have taken rough measurements of the resistance, by the 

 simple process of restoring the original galvanometer deflection 

 by adding or removing resistance coils. A half-inch tube, eight 

 inches long, of selected iron turnings, had a resistance of 2500 

 ohms in the sensitive stale. A feeble stimulus, caused by a 

 distant electrophorous spark, brought it down 400 ohm.s. A 

 rather stronger one reduced it by 500 and 600, while a trace 

 of spark given to a point of the circuit itself, ran it down 

 1400 ohms. 



This is only to give an idea of the quantities. I have not yet 

 done any seriously metrical experiments. 



From the wall diagram which summarises the various de- 

 tectors, and which was prepared a month or so ago, I see I 

 have omitted selenium, a substance which in certain states is 

 well known to behave to visible light as these other microphonic 

 detectors behave to Hertz waves. 



And I want to suggest that quite possibly the sensitiveness of 

 the eye is of the same kind. As I am not a physiologist 1 

 cannot be seriously blam.ed for making wild and hazardous 

 speculations in that region. I therefore wish to guess that some 

 part of the retina is an electrical organ, say like that of some 

 fishes, maintaining an electromotive force which is prevented 

 from stimulating ihe nerves 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 ai'e stimulated. 



1 do not feel clear which part is taken by the rods and 



NO. I2S4. VOL. 50] 



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

 make the hypothesis too definite at present. 



If I had to make a demonstration mode! of the eye on these 

 lines, I should arrange a little battery to excite a frog's nerve 

 and 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 ofgroser waves, I would use a layer of seleniu;ii. 



But the bad contact and the Hertz waves are the most instruc- 

 tive, 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 sensa- 

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

 black is due to the mechanical or tapping-back stimulus. 

 Darkness is physiologically not the mere cessation of light. 

 Uoth are positive sensations, 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 

 ,'(,th second, so as to give the right amount of persistence of 

 impression. 



Eye Model with Electric Bell on BoarJ. 



No doubt in the eye the tapping back is done au'omatically 

 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 b^ard 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 eflfect, 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 tuba is, perhaps, not the best 

 vibrator ; clockwork might do better, because the bell contains 

 in itself a jerky current, which produc.'s one effect, and a 

 mechanical vibration, which produces an opposite effect ; 

 hence the spot of light can hirdly keep still. By lessening 

 the vibration —say by detaching the bell from actual con- 

 tact with the board, the electric jerks of the intermittent 

 current drive the spot violently up the scale ; mechanical 

 tremor brings it down again. 



Vou observe that the eye on this hypothesis is, in elect r.i 

 meter language, heterostatic. Tne energy of vision is sup- 

 plied by the organism, the light only pulls a trigger. Whereas 

 the organ of hearing is idiostatic. I might draw further 

 analogies, about the effect of blows or disorder causing irre- 

 gular conduction and stimulation, of the galvanometer in the 

 one instrument, of the brain cells in the other. 



A handy portable exciter of electric waves is one of the 

 ordinary hand electric gas-lighters, containing a small revolving 

 doubler — i.e., an infuctiveor repl.-nishing machine. A coherer 

 can feel a gas-lighter across a lecture theatre. Minchin often 

 used them for stimulating his impulsion cells. I find that, when 

 held near, they act a Irttle before the spark occurs, plainly 

 because of the little incipient sparks at the brushes or tinfoil 

 contacts inside. A Voss machine acts similarly, giving a small 

 deflection while working up before it sparks. 



And notice here that our model eye has a well-defined range 

 of vision. It cannot see waves loo long for it. 



Ihllz Sparks not e-tciling Tube : e.\upt liy h:lp of a polished 

 knob. 



The powerful disturbance caused by the violent flashes of a 

 Wimshurst or Voss machine it is blind to. If the knobs of the 

 machine are well polished, it will respond to somehigh harmonics, 

 due to the vibrations in the terminal rods ; and these are the 

 vibrations to which it responds when excited by a coil. The 

 coil should have knobs ins ead of points. Sparks from points 

 or dirty knobs hardly excite the coherer at all. But hold a 

 well-polished sphere or third knob between even the dirty 

 knobs of a Voss machine, and the coherer responds at once to the 

 surgings got up in it. 



Electrophorous Lid and ijisulatcd Split le. 



Feeble short s^parksagain are often more powerful exciters than 

 are strong long ones. 1 suppose because they are more sudden. 



This is mstructively shown with an electrophorous lil. Spark 

 it to a knuckle, and it does very little. Spark it to a knob, and 

 it works well. But now spark it to an insulated sphere, there is 

 soaie eflect. Discharge ti.e sphere, and lake a second spark. 



