Geitel determined that there was some common cause as to the discharge of bodies of light and the discharge from the earth’s surface. A series of experiments lasting three years, consisted in investigating the relation of the ultra-violet rays from the sun simultaneously to the quantity of charge in the atmosphere. The results acted as evidence of the explanation of the daily and annual variation of atmospheric potentials. These experiments are of importance in connection with X-rays, because Röntgen and Prof. J. J. Thomson subsequently, and possibly others independently, discovered that X-rays produce, not only a like, but a more extended action in that there is not so great a difference between their power to discharge negatively and positively electrified bodies. § 90_a_. In the further developments of their ideas, they tried the action of diffused day-light upon a Geissler tube traversed by vibrations which were produced by a Hertz vibrator (see recent book on Hertzian waves), the tube having an electrode of metal of the alkaline group. They were able to adjust the combination so that the presence of a little day-light would initiate a luminous discharge, while in the dark such a charge ceased. § 14_a_. 99. ELSTER AND GEITEL’S EXPERIMENT. EFFECT OF POLARIZED LIGHT UPON THE CATHODE. _Berlin Akad._ ’95. _Nature_, Lon., March 28, ’95, p. 514. _Proc. Brit. Asso._, Aug. 16, ’94; Aug. 23, ’94, p. 406.—The X-rays have properties similar to those of light, and have their source in electricity. Quincke discovered that light which has been polarized perpendicularly to the plane of incidence is greatly increased as to its power of penetrating metals. Elster and Geitel used the following apparatus to determine the relation between polarized light and electricity. The current varied according to the angle of incidence and the plane of polarization. The apparatus comprised the following elements: An exhausted bulb, provided with a platinum anode, and a cathode consisting of potassium and sodium, combined in the form of a liquid alloy having a bright surface of reflection. The source of light was an oxyhydrogen flame, which played upon zircon instead of lime; a lens changed the diverging rays to parallel rays, which were polarized by a Nichol prism and allowed to fall upon the cathode. The electrodes of the vacuum bulb were connected to the poles of a generator of a current of about 400 volts. “The strength, of the current was greatest when the plane of polarization was perpendicular to the plane of incidence—__i.e.__, when the electric displacements constituting light, took place in the plane of incidence, and when the angle of incidence was about 60°, __i.e.__, the polarizing angle of the alloy itself.” Prof. Sylvanus P. Thompson confirmed these results by experiment. The rate of discharge was greatest, he said, when the plane of polarization was such that the Fresnellian vibration “chopped into” the surface.