Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S., and E. S. Grew, M.A. 



FEBRUARY, 1915. 



CHAPTERS IN SPECTRUM ANALYSIS. 



By W. MARSHALL WATTS, D.Sc. 



L — Law and Order in Spectra. 

 A. Line Spectra. 



" First the flaming red 

 Sprang vivid forth ; the tawny orange next ; 

 And next delicious yellow, by whose side 

 Fell the kind beams of all-refreshing green ; 

 Then the pure blue, that swells autumnal skies. 

 Ethereal played ; and then, of sadder hue. 

 Emerged the deepened indigo, as when 

 The heavy-skirted evening droops with frost ; 

 While the last gleamings of refracted light 

 Died in the fainting violet away." 



" Science moves but slowly, slowly, creeping on from 

 point to point." 



The scientific investigator is a man with an in- 

 quisitive mind. He may possess the aesthetic 

 temperament which can appreciate to its fullest 

 extent the beauty of any phenomena he observes, 

 but he is never satisfied until he can understand 

 and explain the cause of the phenomenon he in- 

 vestigates ; and he is only completely happy when 

 he has succeeded in reducing the matter to formulae. 

 Others may be satisfied with the perception of 

 beauty, and may regard the attempt to express 

 the phenomena by mathematical formulae as a sort 

 of profanation. I have known musicians impatient 

 of all attempts to explain harmony or concord or 

 discord, and there are artists capable of painting 

 the rainbow with the colours in the wrong order! 

 Can there be anything more beautiful than a 

 spectrum (or, at first sight, more uninteresting 

 than a catalogue of the wave-lengths of the lines 

 which make up the spectrum) ? Yet I venture to 



think that there is, so to speak, more beauty in 

 the law and order revealed by a patient study of a 

 dry catalogue of wave-lengths than that perceived 

 by the mere contemplation of the harmoniously 

 grouped colours of the spectrum visible to the eye. 



" The harmonious spheres 

 Make music, though unheard their pealing 

 By mortal ears." 



Even in the early days of spectrum analysis it 

 was felt that there must be some connection between 

 the vibrations to which the bright lines of a glowing 

 gas are due, and the earlier attempts at tracing 

 this connection were based upon analogy with 

 music. In following up such an analogy we must 

 employ numbers of vibrations instead of wave- 

 lengths. The number of vibrations per second of 

 the red light of incandescent hydrogen is obtained 

 by dividing the velocity of hght, 300,000 kilo- 

 metres per second, by the wave-length, which is 

 6563 ten millionths of a millimetre for the red 

 Hu line. This gives 457-108 million milHon vibra- 

 tions per second. To avoid these unwieldy numbers 

 it is preferred to divide the wave-length into one 

 centimetre, or 10^ ten millionths of a millimetre,* 

 so that we obtain the number of oscillations made 

 while light is travelling one centimetre. This is 

 called the " oscillation-frequency." In the visible 

 portion of the spectrum, X, the wave-length 

 in tenth-metres, is represented by four figures 

 before the decimal point, and lO'/X, or lO^X"' is 



* One-ten millionth of a millimetre is called one " tenth-metre." 



33 



