210 DR. W. GEOFFREY DUFFIELD ON THE 



Where it is stated that a line is nearly symmetrical, but that the absorption line is 

 slightly to the violet of the centre of the broadened line, it is to be understood that it 

 is the geometric centre that is indicated ; it is possible that the positions of maximum 

 emission and maximum absorption are coincident, though this is not necessarily the 

 case, as was demonstrated for certain iron lines. 



We note from the photographs that when a line which is self-reversed is encroached 

 upon by the wing of an adjacent line, it gives rise to an absorption line upon the 

 bright wing. (Cf. 32 on 31.) 



But that when a line which is not self-reversed at a lower pressure is similarly 

 encroached upon it does not reverse. (Cf. 35 and 36 in the wings of 31.) 



It seems probable that this is due to differences in the distribution of the vibrating 

 centres responsible for the different lines in the arc itself, which give rise to 

 self-reversed lines having a different density or temperature gradient from those which 

 do not produce absorption so readily. 



6. Changes in the Relative Intensities of Nickel Lines. With increase of pressure 

 the spectrum undergoes a change which involve some lines becoming relatively more 

 prominent than they were before. It has previously been pointed out that it is very 

 difficult to assign a value to the intensity of a broadened line, because though the 

 area it covers is greatly increased by pressure the intensity per unit area is reduced. 

 The energy due to each line is the important quantity, but unfortunately it could only 

 be determined by an integration which it would be extremely difficult to carry out. 



In the sixth and seventh columns of Table IX. is given an account of the changes 

 which have been observed in the intensities of nickel lines. 



The following are the enhanced lines of nickel given by LOCKYER* : 



384970 4067-30 4245'0 4609'4 



3889'80 4187'8 4279'4 46657 



4015'76 4192-4 4362'3 4679'4 



Only one of these lines has been observed to show a marked change in intensity 

 under pressure, namely, 3889"80 (line 87 upon Plate 2), which is classified as weakened 

 with increase of pressure ; this is the result of its great broadening, and does not 

 necessarily denote any reduction in the total energy emitted by the vibrating centre 

 responsible for it. One other enhanced line, 4067 "30, appears upon the photograph 

 at 10 atmospheres pressure, which has had a prolonged exposure, and it is there 

 strengthened relatively to some of the faint lines near it ; it does not persist at higher 

 pressures ; as none of the other enhanced lines have been observed under pressure, it is 

 the only exception to the general conclusion that pressure does not favour the appearance 

 of enhanced linesf. This is in agreement with previous work upon other spectra. 



* LOCKYBR, ' Report,' Solar Physics Committee. 



t REECE, in a less conservative list of enhanced lines includes 4368 45, here classified as strengthened 

 under pressure, ' Astrophysical Journal,' vol. XIX., p. 334, 1904 ; also 4231-23, here weakened. 



