902 



SCIENCE 



[N. S. Vol. XLIV. No. 1148 



determination of velocities in the line of 

 sight, by the Doppler-Pizeau principle. 

 A large mass of such data has been col- 

 lected, from which some important gen- 

 eralizations have been derived. For ex- 

 ample, Campbell has determined the ve- 

 locity and direction of motion of the solar 

 system through space, and has found a re- 

 markable and as yet unexplained relation 

 between the velocities of stars and their 

 apparent age, the redder and presumably 

 older stars and a class of nebula? having 

 in general the greater velocities. It like- 

 wise appears that two immense star streams 

 are crossing each other in the Milky "Way. 

 Many spectroscopic binaries have been dis- 

 covered and their orbits determined, and 

 recently there have been found remarkable 

 displacements and rotations in nebulae 

 which may throw some light on the nature 

 and destiny of these bodies. The spectro- 

 scope has enabled astronomers to under- 

 take the ambitious task of tracing the 

 course of stellar evolution. 



The most ingenious and fruitful device 

 for studying the sun is the speetrohelio- 

 graph, invented by Hale in 1892. With 

 this instrument photographs of the distri- 

 bution of a given constituent of the solar 

 atmosphere may be obtained by restrict- 

 ing the light falling on the photographic 

 plate to the wave-length of one of the 

 characteristic lines of the element. The 

 configuration of the hydrogen clouds in the 

 neighborhood of sunspots led Hale to sus- 

 pect vortical motions in such regions. In 

 1908 the study of a number of plates, 

 which showed that hydrogen flocculi were 

 actually drawn into these spots from great 

 distances, proved without question that 

 sunspots are cyclonic areas of enormous 

 extent. Thus the long-disputed question 

 as to the nature of sunspots was answered, 

 but this was not all. Vapors which emit 

 or absorb line spectra are ionized, and as 

 the more mobile electrons would diffuse 



more rapidly to higher levels than the posi- 

 tive ions, Hale inferred that the immense 

 whirls of electrified vapors in the neigh- 

 borhood of the spots must cause a radial 

 magnetic field. If such fields are suffi- 

 ciently intense, the longitudinal Zeeman 

 effect should be produced. As a matter of 

 fact, the spectrum of light from the spots 

 is characteristically different from that of 

 the surrounding photosphere, one of these 

 peculiarities being the doubling of many 

 lines. As Hale anticipated, an examina- 

 tion of the state of polarization of such 

 lines showed them to be circularly polar- 

 ized, and the direction indicated that the 

 whirling vapor was negatively electrified. 

 Hale likewise sought for the more minute 

 effects which might be expected from the 

 rotation of the solar atmosphere as a 

 whole. A study of the breadth of spectral 

 lines at different latitudes and the detec- 

 tion of traces of circular polarization at 

 their edges showed that the sun possesses 

 a magnetic field with polarity correspond- 

 ing to that of the earth, but of much 

 greater intensity. Although the atmos- 

 pheric conditions on the earth are very 

 different from those on the sun, it is pos- 

 sible that these investigations may assist 

 us in solving the baffling problem of the 

 earth's magnetism. 



One of the most impressive facts re- 

 vealed by the spectroscope is the substan- 

 tial identity of constitution of the heavenly 

 bodies. Everywhere we find evidence of 

 the existence of such elements as hydrogen, 

 sodium, calcium and iron. But we also 

 find an infinitude of differences in the ap- 

 pearance of the lines, which we must at- 

 tribute to differences of temperature, 

 vapor density, pressure and electrical con- 

 dition. It is suggestive to find that the 

 spectrum of some stars resembles that of 

 the arc, of others that of the spark. We 

 may hope by comparing the spectra of 

 these bodies with those produced in our 



