---------------------------------------------------------------------- DETECTORS OF RADIATION. --------+---------+------------+-------------+-----------+------------ Physio- |Chemical.| Thermal. | Electrical. |Mechanical.|Microphonic. logical.| | | | | --------+---------+------------+-------------+-----------+------------ | | | | |Selenium.(?) | | | | | Eye. |Photo- |Thermopile. | Spark. | Electro- | Impulsion | graphic | | (Hertz.) | meter. | Cell. | Plate. | | |(Blyth and | (Minchin.) | | | | Bjerknes.)| | | | | | ˟Frog’s |Explosive| Bolometer. | Telephone; | Suspended | Filings. Leg | Gases. |(Rubens and | Air-gap | Wires. | (Branly.) (Hertz | | Ritter.) | and Arc. |(Hertz and | and | | | (Lodge.) | Boys.) | Ritter.)| | | | | | | | | | |Photo- | Expanding |Vacuum Tube. | | Coherer. | electric| Wire. | (Dragoumis.)| |Hughes and | Cell. |(Gregory.) |Galvanometer.| | Lodge. | | Thermal |(Fitzgerald.)| | | | Junction. |Air-gap and | | | |(Klemencic.)|Electroscope.| | | | | (Boltzmann.)| | | | |Trigger Tube.| | | | |(Warburg and | | | | | Zehnder.) | | --------+---------+------------+-------------+-----------+------------ ˟ 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.