[Illustration: Syntonic Sender and Receiver used in the experiments plotted on page 28. The switch enables the coherer K to be connected either to the tuned resonator M L N or to the detecting circuit E F. Weak impulses are felt when the switch is C E, D F; strong impulses when the switch is C A, D B; provided the coil L is similar to the coil of the radiator above. The impulses are plotted in the diagram Fig. 19A.] [Illustration: _October 27, 1897._ FIG. 19A.--Current through Coherer after successive Electrical Stimuli, without any mechanical tapping back. The sudden rises are obtained when the circuits are syntonised. Weaker stimuli cause the descents.] The fluctuations of resistance of a coherer dependent on various strengths of stimulus are instructively shown in some metrical experiments made by Mr. Robinson, and a plotting of which I showed to the Physical Society of London in 1897. This plotting is here reproduced, and it shows the singular fact that, whereas a stronger electrical stimulus usually decreases the resistance, as is natural, a weaker subsequent stimulus usually increases it again: so that alternately strong and weak stimuli send the curve zigzagging up and down, until it gets into a condition demanding rejuvenation by a mechanical tap back. Sometimes a decidedly strong electrical stimulus knocks down the conductivity of the coherer as if it had been tapped back. This is almost certainly due to a burning of the delicate contacts--a blowing of a fuse as it were,--and the effect of this electrical burn back is quite different from the effect of a mechanical tap back, inasmuch as it leaves the coherer insensitive. A shaking up is necessary to restore it. I now call your attention to the Table on next page of various kinds of detector for electric radiation distributed in groups. 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._