28o PHYSICAL SCIENCE 



as can be expressed the connection between a 

 musical note and its overtones. Two or even 

 three series of Hnes may exist, and two or three 

 modifications of the formulae are then needed to 

 co-ordinate their vibration numbers. That such 

 distinctions possess a physical significance was 

 shown by experiments of Lenard, who found 

 that the three series of sodium and lithium lines 

 are separated in the flame of the electric arc, 

 the outer shell of flame giving only the funda- 

 mental series, while, in the physical conditions 

 appertaining to the inner flame, the second and 

 third series become dominant. 



We have already dealt in Chapter VIII. with 

 the importance of these series of spectral lines 

 in the problem of atomic structure, and shown 

 how they enabled Bohr to explain the emission 

 of radiation in finite units as required by Planck's 

 quantum theory. 



Some most interesting work relating to the 

 sun was carried out by means of Professor Hale's 

 method of photographing the sun itself and its 

 prominences by the light corresponding with one 

 definite spectral line. Two of the commonest 

 elements present in the sun are hydrogen and 

 calcium, and these elements are marked by the 

 strong lines h and k respectively. The resultant 

 photographs, then, show the distribution of 

 hydrogen or calcium throughout the region in- 

 vestigated. The spectra of the prominences at 

 the edge of the sun's disc consist of bright lines, 

 while some of the dark absorption lines of the 

 light from the surface of the sun possess bright 

 centres, like those shown in Fig. '^'], indicating 



