530 WRIGHT— RECENT SPECTROSCOPIC OBSERVATIONS. 



trically active stars, attention may be directed to a rather remarkable 

 continuous spectrum which begins near the hmit of the Balmer 

 series of hydrogen and extends toward the uhra-violet. The spec- 

 trum may be seen in the photograph (Fig. 9) as a broad, faint band 



Fig. 9. 



lying to the left of the strong negular image. ^ A similar phenome- 

 non has been observed by Evershed in the solar chromosphere. The 

 spectrum is assumed to be due to hydrogen, though nothing of the 

 sort has been found, with certainty, in the laboratory. It is perhaps 

 significant that a spectrum would be expected there if we accept the 

 Bohr atomic theory. From the atomic model which Bohr sets up, 

 the Balmer line series of hydrogen develops from the recovery of a 

 partially separated electron, while an extension of his equations 

 to include the capture of free electrons by positive nuclei estab- 

 lishes a continuous spectrum at just this place. I must confess 

 that I venture into the domain of the physicist with trepidation, and 

 I have, for the purposes of this small excursion, sought the hospi- 

 table protection and guidance of Professor Millikan, which have 

 been generously accorded. Professor Millikan has pointed out that 

 the justification of this interpretation would depend upon the ratio 

 of the energy of agitation of the electron to the energy expended 

 in capture, that is to Planck's constant multiplied by the frequency 

 of vibration at the limit of the Balmer series. A temperature of 

 about 6000° centigrade would afiford the requisite amount of kinetic 

 energy. As a matter of fact that is about the temperature of the 

 solar chromosphere, which, as we have seen, also emits this spec- 

 trum. It will be recalled in this connection that Buisson, Fabry, and 

 Bourget have estimated the temperature of the Orion Nebula to be 

 about 15000° Cent. It seems equally possible that the electronic 



^ This illustration, unlike the others, shows a spectrum recorded with a 

 " slitless " spectograph. For this reason a bright line is represented by an 

 image of the nebula, instead of by a narrow line, as in the other illustrations. 



