Results:--The resistance of a Geissler tube provided with a cathode surface of pure alkali metal is diminished by the light from the sparks of an induction coil; especially when the pressure is ·1 to ·01 mm. of mercury. The resistance which rarefied gas opposes to an electric current in a magnetic field is greatest in the direction normal to the magnetic lines. The changes of resistance effected by any kind of light in a vacuum tube with alkali metal cathode can be measured galvanometrically. (A Daniell cell gives 100 divisions on a Rosenthal galvanometer when coupled up through such an illuminated tube, each division meaning about 10⁻¹⁰· ampere.) [Illustration: FIG. 64. _Explanation of Fig. 64._--A vacuum tube of rarefied hydrogen containing alkali metal as cathode, say the liquid K-Na alloy, or solid K or Na. A spark gap at S serves as alternative path, and a stream of sparks can occur to the plate P in the dark. But when light falls on the surface A this stream of sparks can cease, showing that the resistance of the vacuum tube is diminished.] [Illustration: FIG. 65. _Explanation of Fig. 65._--Showing position of magnetic poles with respect to the vacuum tube discharge. With the poles _across_ the line of discharge, as in Fig. on left, excitation of the magnet opposes the leak from the surface. With the poles as in Fig. on right, the discharge is not much affected, it is even sometimes slightly increased.] [Illustration: FIG. 66. _Explanation of Fig. 66._--Potassium vacuum bulbs containing ⅓ millimetre of hydrogen mounted and connected to battery and galvanometer, and arranged as a photo-electric photometer.] _Wied. Ann., 48, p. 625. On the Photo-Electric Comparison of Sources of Light._ Attempts to make such a potassium cell into a photometer. _Wied. Ann. 52, p. 433. Further Photo-Electric Experiments._ Plates of platinum, silver, copper need exceedingly ultra-violet light before they show any photo-electric power; zinc, aluminium, magnesium show it for visible violet and blue light; the alkali metals, in an atmosphere of rarefied hydrogen, advance their range of sensibility into the spectral red; while under the most favourable conditions they show a sensibility only inferior to that of the eye itself. The authors now use galvanometric methods of measuring the effect, instead of only electrometers, and they arrive at the following results:-- (1) The three alkali metals Na, K, Rb, have different sensibility for differently-coloured lights. For long waves their order of sensibility is Rb, Na, K; though rhubidium is far exceeded by the other two metals in white light. (2) Illumination of a plane alkali metal cathode surface with polarised light causes greatest discharge if the plane of polarisation is normal to plane of incidence; and least, if the two coincide.