208 DE. W. GEOFFKEY DTIFFIELD ON THE 



possible is a modification of that originally used by HUMPHREYS,* and it has already 

 been described.! 



4. The Broadening of the Lines : (l) General Features. -The general phenomenon 

 of the broadening of lines under pressure has been described elsewhere. In the Nickel 

 Spectrum : 



(a) Some lines broaden nearly symmetrically ; but 

 (6) Most lines broaden unsymmetrically. 



Of the latter class by far the larger number are more extended towards the red end 

 of the spectrum, but a few are unmistakably broadened more on the violet side. 



It is usually possible to distinguish between two classes of lines at atmospheric 

 pressure : namely those that are sharp and those that are soft or nebulous. 



Under pressure some few of the former retain some of their characteristic hardness 

 of outline at moderate pressures (10 to 20 atmospheres), while others lose their sharp 

 appearance altogether and quickly become nebulous. Many that start by being 

 nebulous become more so and disappear. 



Measurements of the broadening of the lines have not been made, but some 

 indication of their relative behaviour in this respect is given in the first row of the 

 eighth column of Table IX., which classifies them in the following order of increasing 

 width : slight, s ; moderate, m ; considerable, c ; great, g ; very great, vg ; very very 

 great, G. 



The other columns give some account of the nature of the broadened line, whether 

 shai-p or nebulous, symmetrical or unsymmetrical. The abbreviation b s indicates that 

 the broadening is nearly symmetrical (very few are quite symmetrical, there is usually 

 a slightly larger wing on the red side), b r indicates that the broadening is greater 

 towards the red, and b v that it is greater on the violet side. It is impossible to 

 examine the broadening of lines in any detail without being impressed by the 

 inadequacy of the nomenclature of the spectroscopist. Under pressure a "line" is 

 but a courtesy title for the extended patch of luminosity into which its original sleek 

 proportions have degenerated, and it is valueless as a description. Nor is the term 

 band quite appropriate, since a banded spectrum is by common usage something rather 

 different in appearance from that of a spectrum ordinarily produced by pressure. 

 With reluctance I have retained the word line. 



The terms sharp and nebulous are usually employed to distinguish between the 

 two well-known types of spectrum lines, but the former does not seem quite 

 satisfactory when applied to a line whose energy has been diffused over several 

 Angstrom Units. Nevertheless a distinction is to be discerned even under pressure in 

 the appearance of the lines, which it is valuable to make, and I have adopted the 

 usual terms. The origin of the difference seems to lie in the shape of the intensity 

 curves these are gradual at the boundaries of those lines which are usually called 



* HUMPHREYS, ' Astrophysical Journal,' vol. VI., p. 169, 1897. 

 t DUFFIELD, 'Phil. Trans.,' A, vol. 208, p. 117, 1908. 



