Selenium is inserted in this table in the microphone column, because it is a substance which in certain states is well known to behave to visible light as these other microphonic detectors behave to Hertz waves. It is inserted with a query, to indicate that its position in the table is not certainly known. It may possibly belong to some other column.
Electrical Theory of Vision.
And I want to suggest that quite possibly the sensitiveness of the eye is of the coherer kind. As I am not a physiologist, I cannot be seriously blamed 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 the nerves solely by an intervening layer of badly conducting material, or of conducting powder 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, though I hope it is obvious what I intend to suggest.
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 physiologically not the mere cessation of light. Both 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, ⅒th second, so as to give the right amount of persistence of impression.
| DETECTORS OF RADIATION. | |||||
|---|---|---|---|---|---|
| Physiological. | Chemical. | Thermal. | Electrical. | Mechanical. | Microphonic. |
| Selenium.(?) | |||||
| Eye. | Photographic Plate. | Thermopile. | Spark. (Hertz.) | Electrometer (Blyth and Bjerknes.) | Impulsion Cell. (Minchin.) |
| ˟Frog’s Leg (Hertz and Ritter.) | Explosive Gases. | Bolometer. (Rubens and Ritter). | Telephone; Air-gap and Arc. (Lodge.) | Suspended Wires. (Hertz and Boys.) | Filings. (Branly.) |
| Photoelectric Cell. | Expanding Wire. (Gregory.) Thermal Junction. (Klemencic.) | Vacuum Tube. (Dragoumis.) Galvanometer. (Fitzgerald.) Air-gap and Electroscope. (Boltzmann.) Trigger Tube. (Warburg and Zehnder.) | Coherer. Hughes and Lodge. | ||
˟ The cross against the frog’s leg indicates that it does not appear really to respond to radiation, unless stimulated in some secondary manner. The names against the other things are unimportant, but suggest the persons who applied the detector to electric radiation.
The interrogation mark against Selenium indicates that its position in the microphonic column may be doubtful.
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.