A similar explanation will apply to the positive rays discovered by Villard (_Comptes rendus_, 143, p. 674) and J. J. Thomson (_Phil. Mag._ 13, p. 359), which travel in the opposite direction to the rays we have been considering, i.e. they travel away from the cathode and in the direction of the cathode's rays; these rays are sometimes called "retrograde" rays. These as far as has been observed have always the same maximum value of e/m, i.e. 10^4, and there are a considerable number of negative ones always mixed with them. The maximum velocity of both the positive and retrograde rays is about 2 × 10^8 cm./sec. and varies very little with the potential difference between the electrodes in the tube in which they are produced (J. J. Thomson, _Phil. Mag._, Dec. 1909). The positive rays show, when the pressure is not very low, the line spectrum of the gas through which they pass. An exceedingly valuable set of observations on this point have been made by Stark and his pupils (_Physik. Zeit._ 6, p. 892; _Ann. der Phys._ 21, pp. 40, 457). Stark has shown that in many gases, notably hydrogen, the spectrum shows the Doppler effect, and he has been able to calculate in this way the velocity of the positive rays. _Anode Rays._--Gehrcke and Reichenhein (_Ann. der Phys._ 25, p. 861) have found that when the anode consists of a mixture of sodium and lithium chloride raised to a high temperature either by the discharge itself or by an independent heating circuit, very conspicuous rays come from the anode when the pressure of the gas in the discharge tube is very low, and a large coil is used to produce the discharge. The determination of e/m for these rays showed that they are positively charged atoms of sodium or lithium, moving with very considerable velocity; in some of Gehrcke's experiments the maximum velocity was as great as 1.8 × 10^7 cm./sec. though the average was about 10^7 cm./sec. These velocities are less than those of the positive rays whose maximum velocity is about 2 × 10^8 cm./sec. (J. J. T.) FOOTNOTES: [1] The values for nickel and bismuth given in the table are much higher than later values obtained with pure electrolytic nickel and bismuth. [2] The value here given, namely 12.885, for the electric mass-resistivity of liquid mercury as determined by Matthiessen is now known to be too high by nearly 1%. The value at present accepted is 12.789 ohms per metre-gramme at 0° C. [3] The value (1630) here given for hard-drawn copper is about ¼% higher than the value now adopted, namely, 1626. The difference is due to the fact that either Jenkin or Matthiessen did not employ precisely the value at present employed for the density of hard-drawn and annealed copper in calculating the volume-resistivities from the mass-resistivities.