24 
GEOPHYSICS: L. A. BAUER 
ceptionally profuse emission shown by potassium and sodium when the 
electric vector of the light wave vibrates normally to the surface. But 
so far as we can discover the selective effect in lithium has never been 
shown to possess this characteristic. 
Summary. — 1. Photo-electric phenomena do not appear to be in gen- 
eral conditioned by the presence of a gas, since they appear with the 
use of sufficiently short wave lengths even when they do not appear 
with the use of long wave lengths. 
2. The long wave-length limit of photo-sensitiveness is modified by 
the presence of a gas and this long wave-length Hmit is determined by 
the presence of a substance which gives rise to a maximum photo- 
sensitiveness at the so-called resonance wave-length. 
3. Some of the characteristics of the selective effect have been obtained 
under the conditions under which only the normal effect is commonly 
observed. 
4. The curves commonly supposed to be characteristic of the normal 
effect may be made to merge by insensible gradations into those of the 
selective effect by the action of gas on the surface of the metal. 
5. All distinctions between the normal and selective effects in lithium 
have disappeared. 
1 K. Fredenhagen, Physik. Zs., Leipzig, 15, 65 (1914). 
2 H. Kiister, Ihid., 15, 68 (1914). 
3 G. Wiedmann and W. Hallwachs, Berlin, Ber. D. physik. Ges., 16, 107 (1914). 
4 R. A. Millikan and W. H. Souder, Physic. Rev., Ser. 4, No. 1, 13, July, 1914. 
5 R. Pohl and P. Pringsheim, Die lichtelektrischen Erscheinungen, pp. 14 and 27. 
« G. Wiedmann, Ber. D. Physik. Ges., 17, 343 (1915). 
7 R. A. Millikan, Physic. Rev., March, 1916. 
8 R. Pohl and P. Pringsheim, Ber. D. Physik. Ges., 14, 546 (1912), 
» R. Pohl and P. Pringsheim, Ibid., 46 (1912). 
CONCOMITANT CHANGES IN TERRESTRIAL MAGNETISM 
AND SOLAR RADIATION 
By L. A. Bauer 
DEPARTMENT OF TERRESTRIAL MAGNETISM. CARNEGIE INSTITUTION OF WASHINGTON 
Read before the Academy, November 17, 1915. Received December 3, 1915 
While good progress has been made by various investigators in estab- 
lishing the relationship between fluctuations of the earth's magnetism 
and those of the sun's activity during the sun-spot cycle, there are still 
outstanding a number of important questions. The magnetic quantity 
hitherto generally used, as for example, one of the magnetic elements 
(chiefly the magnetic declination), or the range of the diurnal variation 
