190 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1923 



the apparent variation of chemical composition with stellar evolu- 

 tional progress, are considered. But the chief significance of the 

 adoption of the spectroscope in the observatory lies in the extraord- 

 inary versatility of this instrument, and the possibilities it affords 

 of utilizing in astronomy the widest variety of physical and chemi- 

 cal discoveries. 



In 1842 Doppler tried to prove that the color of a star depends 

 upon its velocity. If a star radiated monochromatic light and its 

 velocity were great enough, his conclusion would be correct. Rightly 

 applied with the spectroscope, his principle has given us the means 

 of measuring the motions of gases in the solar atmosphere ; the rota- 

 tion of the sun, planets, and nebulas; the orbital velocity of close 

 double stars discoverable only by this method; and the velocity in 

 the line of sight of various celestial objects. 



I wish that time permitted me to dwell on the extraordinary har- 

 vest which has resulted from the skillful application of this and 

 other principles of physics, but I can only recall a few of them. 

 The shift toward red or violet of spectral lines by pressure affords 

 a means of measuring the pressure in stellar atmospheres, after 

 other effects have been allowed for. The variation of the relative 

 intensities of lines with temperature gives one clue to stellar tem- 

 peratures, and lias also led indirectly to Adams' beautiful method of 

 deriving absolute magnitudes and parallaxes from stellar spectra. 

 Reduced to a sound scientific basis through the recent advances of 

 physics, the study of line intensities has also become one of our most 

 powerful guides, not only to the nature of stars but to the structure 

 of the atom itself. The shift of the maximum of intensity in the 

 spectrum as a function of the temperature, the influence of magnetic 

 and electric fields on radiation, the phenomena of polarization, of 

 anomalous dispersion, and of optical resonance are also among the 

 numerous discoveries of the physicist, which the astronomer has al- 

 ready utilized, with important positive or negative results. 



In addition to the spectroscope, the astronomer has derived from 

 the physical laboratory a long line of other valuable instruments. 

 The photometer, now powerfully supplemented and largely dis- 

 placed by photographic methods, has given us the magnitudes of tens 

 of thousands of stars. The thermopile, bolometer, and radiometer 

 have led to remarkable advances in our knowledge of the infra-red 

 spectrum, the precise measurement of the varying intensity of the 

 solar radiation, the determination of the heat radiation of stars as 

 faint as the thirteenth magnitude, and even to studies of the energy 

 spectra of some of the brighter stars.' The photo-electric cell has 

 yielded stellar photometric measures of surprising precision. The 

 radiometer, which gave the first actual measure of the pressure of 

 radiation, now known to play such a dominant part in the massive 



