38 



using the laboratory data, some lines being shifted 

 by pressure to the red and others to the violet. It 

 is found that some of the solar gases are rising, 

 while others are falling. The pressure at different 

 heights is then determined, and found to range 

 from about an atmosphere close to the solar surface 

 to exceedingly low values, such as we know in 

 vacuum tubes, at elevations of several thousand 

 miles. The delicacy of this method is illustrated 

 by the fact that the spectrographs on Mount 

 Wilson and in Pasadena show a distinct difference 

 in the position of certain lines in the electric-arc 

 spectrum, caused by the difference in atmospheric 

 pressure between mountain and valley. Thus 

 we are enabled to sound the solar atmosphere 

 through all its depths and to learn of its phenom- 

 ena at different levels. The same method, when 

 applied to stars, has given us a preliminary deter- 

 mination of the pressure in stellar atmospheres. 



THE "FLASH" SPECTRUM WITHOUT AN ECLIPSE. 



In designing the first tower telescope, one of the 

 objects in view was to provide a means of photo- 

 graphing the 'flash' spectrum without the aid 

 of a total eclipse. When the moon passes between 

 the earth and the sun it cuts off the brilliant light 

 of the solar disk and permits the spectrum of its 

 gaseous atmosphere to be photographed. The 

 narrow arc of light, coming from this luminous 

 atmosphere at the moment when the sun's disk 

 is completely covered by the dark body of the 



