208 DR. W. GEOFFREY DUFFIELD ON THE 



To facilitate reference to the lines arbitrary letters have been assigned to them, 

 beginning alphabetically at the more refrangible end ; a is not included in the portion 

 reproduced, but its behaviour resembles that of the strong line d. 



The prominent features are : 



(1) The broadening of the lines, 



(2) Their displacement towards the red end of the spectrum, 



(3) The changes in relative intensity, 



(4) The obliteration of the series lines within the region examined, 



(5) The absence of reversals. 



Unfortunately the exposures were not sufficiently equal for a strict comparison of 

 the reproduced photographs with one another to be feasible; but Plates 10 and 11 

 illustrate the remarkable fact that at the highest pressures the lines d, Plate 10, and 

 I, o, Plate 11, fail to impress the photographic plate, though they were originally 

 amongst the strongest lines in the copper spectrum. 



The plates also show that the most intense portion of each line is displaced under 

 pressure from the position it occxipies at normal pressure, the direction being that of 

 increasing wave-lengths. Line^', at 70 and 80 atmospheres, affords a good illustration 

 of this point, and, since this line is, when under pressure, completely on the right-hand 

 side of the normal line, the phenomenon cannot be referred to an unsymmetrical 

 broadening. 



Unlike the spectrum of the iron arc in this region, no reversals have been observed 

 under pressure. 



5. THE BROADENING OF THE LINES. 

 From the photographs we learn the following facts : 

 In the region studied (X 4000 to X 4600) 



1. All lines broaden under pressure. 



2. The broadening increases with the pressure, but different amounts of exposure 

 necessarily make it difficult to determine if the increase is continuous and linear with 

 the pressure. 



3. The broadening of all the lines examined is unsymmetrical, being greater on the 

 less refrangible side. 



4. The lines may be divided into two classes according to the nature of their 

 broadening. Those of the first class become under low pressures so faint and hazy 

 that they almost resemble bands (for example, d at 15 and 20 atmospheres, Plate 10) 

 and under higher pressures are dissipated. Those of the second class, though very 

 broad, remain more or less well-defined lines (Plate 10, i, Plate 11, j, n, p, q), some, 

 however, diminishing in intensity until they fail to impress the sensitive plate (I, o, 

 Plate 11). 



5. Those lines which are originally strongest are not necessarily the most broadened 



