ASTRO-PHYSICS 281 



the existence of masses of luminous vapour lying 

 above the reversing layer. These bright central 

 lines give sufficient light for the purpose we are 

 now considering, and the resulting photographs 

 show the distribution of glowing clouds of vapour 

 in the higher regions of the solar atmosphere. 

 Even the dark absorption lines are only dark 

 by comparison with the brighter background, 

 and thus new photographs can be taken with 

 the darker sides of these reversed lines. The 

 light then used comes from a deeper layer in the 

 solar atmosphere, and as many as three calcium 

 photographs have been taken in this way from 

 a single line, showing the distribution of calcium 

 at three different levels in the sun's envelope. 



The method by which Professor Hale obtains 

 these wonderful results consists in the employ- 

 ment of a spectro-helioscope possessing two slits. 

 The solar light is focussed into an image by the 

 telescope, passed through one of these slits, and 

 thrown on to a prism or grating. The spectrum 

 thus produced shows the usual lines, and the 

 second slit is fixed so as to coincide with the 

 line by the light of which the sun is to be 

 photographed. The light coming through the 

 second slit is thus monochromatic light — simple 

 light of the particular kind desired. The first 

 slit is made to travel slowly over the disc of 

 the sun, and the second slit, by appropriate 

 movements, is kept constantly in position to allow 

 the particular line to fall upon it. In this way 

 a complete picture of the calcium or hydrogen 

 flames above the surface of the sun can be 

 obtained. 



One of the striking features of the photo- 



