77 



FIG. 10. 



A diagram of the visible lines of the spectrum of the elements Radium 

 and Helium when rendered incandescent by electric ' sparking ' in a glass 

 tube : kindly prepared for this book by Mr. Frederick Soddy of the 

 University of Glasgow. The position of the chief great lines of the solar 

 spectrum are marked on the lowest horizontal line. On the upper line 

 the wave-lengths of the rays occupying the position indicated, are given. 

 The figure 72 means that the wave-length of the ray occupying this 

 position when refracted by the prism of the spectroscope is, as measured 

 from crest to crest of the undulation, seven hundred and twenty millionths 

 of a millimetre. It is generally written 720-0 pp. 



Lines exist at the ultra-violet end of the spectrum which can be 

 photographed but do not affect the eye that is to say are invisible. On 

 the other hand the lines of the red end of the spectrum do not produce a 

 photographic effect. Consequently a 'photographed' spectrum such as 

 that given in the next figure (fig. n) differs in the lines presented both at 

 the red and the violet ends from the visible series of lines. The two 

 (visible and photographed spectra) agree only from wave-length 587-6 /*/* 

 to wave-length 447-2 pp. 



The two spectra given in fig. 10 show how great is the difference in the 

 position and number of the bands of Radium and Helium yet as shown 

 in the next figure (fig. u) the 'emanation' from Radium actually is 

 transformed into Helium. 



