236 DR. W. GEOFFREY DUFFIELD ON THE 



of previous investigation by the writer ; the copper arc spectrum* yielded the result 

 that the displacement was proportional to a power of the wave-length " at least as 

 great as the third power and possibly as high as the sixth." The experiments upon 

 the gold arc under pressuret favoured a dependence upon third power of the wave- 

 length. 



Subsequent important experiments by GALE and ADAMS} upon an extended region 

 of the iron spectrum under a pressure of 9 atmospheres support this conclusion, and so 

 do the results of the present research. 



The bearing of this upon the spectra of novae has been discussed by the writer in 

 a paper entitled " The Spectra of Novae and the Pressure Effect. " It is there shown 

 how the Doppler and pressure effects may be distinguished. 



An interesting feature of the nickel lines is the gregarious tendency of lines with 

 large displacements, almost all of which occur within a region of the spectrum 

 between X = 4200 and X = 5000. Reference to Table X. which gives lists of lines of 

 similar appearance reversed lines, nebulous lines, &c., indicates that they occupy only 

 limited regions of the spectrum. A similar tendency for the iron lines of the same 

 group to congregate was recorded. 



8. Resolution of the Nickel Spectrum into Groups. -The lines may be grouped in 

 different ways according to their displacement, broadening, intensity, reversal, &c., 

 but of these the most important is the first-named. It has already been seen in the 

 previous section that it is possible to resolve the nickel arc spectrum into two main 

 groups according to their displacement, and that the first of these may be capable of 

 further sub-division. 



The following table has been compiled from the available data to indicate the 

 general nature of the lines belonging to the two groups. 



The line and its wave-length are given in the first two columns, in the third column 

 " n " denotes that the line was nebulous at atmospheric pressure, in the fourth column 

 N indicates that it became nebulous or diffuse under increasing pressure ; " sh " in 

 the next column indicates that the line was classified by HASSELBERG as " sharp " at 

 atmospheric pressure. The changes in relative intensity are given by " wk " or " str " 

 in the next two columns. The broadening and reversal are treated in the two following 

 columns, and the last two indicate the group into which the line falls in the d/\ 3 

 diagrams, Diagram 6, fig. 4, and Diagram 7, fig. 2. The first of these is the more 

 reliable as it includes a larger number of readings, but the second is useful in 

 supplementing the information supplied by the preceding column ; it is derived from 

 observations made at 10 atmospheres only. 



* DUFFIELD, 'Phil. Trans. Roy. Soc.,' vol. 209, p. 205, 1908. 

 t DUFFIELD, 'Phil. Trans. Roy. Soc.,' vol. 211, p. 33, 1910. 



J GALE and ADAMS, ' Astrophysical Journal,' vol. XXXV., p. 10, 1912. Through a misapprehension, 

 these authors quote experiments upon vanadium by Rossi as supporting this conclusion. 

 DUFFIELD, 'Monthly Notices, R.A.S.,' 73, p. 631, 1913. 



