454 ASTROPHYSICS 



indications are probably correct for the average temperature of the 

 contents of the tube, but hardly so for those molecules which are 

 glowing. It has been suggested that perhaps a very small proportion 

 of the molecules receive and carry the discharge: that while the 

 molecules in action may be very hot, the average for all in the tube 

 is very low. It seems reasonable to suppose, also, that the low-tem- 

 perature indication is due to the fact that the current is actually 

 passing but a small fraction of the time. The effect upon the eye 

 is that of a continuous glow, whereas the thermometer measures the 

 average effect. 



The influence of a magnetic field 1 upon the character of spectral 

 lines, established in the laboratory by Zeeman, has not yet been ob- 

 served in celestial spectra, but its detection may be merely a question 

 of the dispersive power available on faint spectra. 



It will be perceived that the interpretation of celestial spectra must 

 be made with circumspection. We are not always justified in reaching 

 conclusions upon the spectroscopic evidence alone; general conditions 

 must also be taken into account. For example, shall we say that the 

 temperature of the gaseous nebulae is very high, because they have 

 bright-line spectra? On the contrary, the difficulty of maintaining a 

 high temperature in a mass so attenuated should be given at least 

 equal weight. The radiating molecules or particles may for the instant 

 be quite hot, but the effective temperature of the whole nebula 

 is probably low. 2 



The experimental verification of radiation pressure by Lebedew 3 

 and by Nichols and Hull 4 is far-reaching in its consequences. We 

 must take this force into account, as truly and as constantly as we 

 must consider gravitation. Radiation pressure requires us to recon- 

 struct our theories of comets' tails, of the corona, of the zodiacal light, 

 of the aurorse, in fact, of every phenomenon of nature involving 

 minute particles. 5 And what celestial object does not involve them? 



On the other hand, the student of the stars has pointed the way 

 for the laboratory investigator, in many instances. The ultra-violet 

 hydrogen series 6 was photographed by Huggins, in the spectrum of 

 Vega, before it was found in the laboratory; and Pickering has dis- 

 covered another hydrogen series, 7 in Zeta Puppis, which still awaits 

 terrestrial duplication. The hypothetical element, helium, in the sun, 

 waited a quarter-century for Ramsay's discovery, 8 and the laboratory 

 investigation 9 of its more complete spectrum which followed. Stu- 



1 Handbuch der Spectroscopie, n, 613-672. 



2 Lehrbuch der Kosmischen Physik, 43. 



3 Annalen der Physik, 1901, vr, 433. 



4 Astrophysical Journal, xv, 62; xvrr, 315; xvii, 352. 



6 Arrhenius, Physikalische Zeitschrift, November, 1900. 



6 Philosophical Transactions, CLXXI, 669. 



7 Astrophysical Journal, V, 92. 



8 Nature, LXV, 161. 



9 Astrophysical Journal, in, 4. 



