EFFECT OF PRESSURE UPON ARC SPECTRA. SILVER. 37 



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

 beginning alphabetically at the more refrangible end. 

 The prominent features are : 



(1) The broadening of the lines ; 



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



(3) The structure which Incomes apparent in the wings of the strong lines under 



pressure ; 



(4) The gradual disappearance of the line spectrum ; 



(5) Its replacement by a banded spectrum ; 



(6) The development of the banded spectrum into an almost continuous 



spectrum. 



The lines seen on the photographs are : 



a 4055 '44 1st sub-series. 



[6 . . .' . . 4058-04 Lead.] 



c 4212'! 1st sub-series. 



d 4311-28 



e 4476-29 2nd sub-series. 



(a) 4055-44. Silver. 1st subordinate series. 



At normal atmospheric pressure it is broad and covers about 13 A.U. It is 

 unsymmetrically reversed, its reversed portion being slightly on the violet side of the 

 centre of the bright line. At 5 atmospheres* the line spreads over 17 A.U. or more, 

 and is so broad that it might almost be called a band. The reversed part is badly 

 defined, and its centre cannot be determined. Its wings now present a marked 

 structure, and are resolvable into a number of well-defined lines, in which there is no 

 obvious regularity. The shadings on the two wings are different from one another. 

 At 20 atmospheres the line has spread out over a greater range and can be distin- 

 guished on the red side as far as 24 A.U. from its original position. The structural 

 appearance, which extends as far as the wings are visible, is well marked, though the 

 individual lines are broader. The broadening is much greater on the red side. At 

 25 atmospheres the line spectrum of silver has almost disappeared and given place to 

 a banded spectrum. Between 25 and 200 atmospheres, this line is indistinguishable 

 against this background of banded or continuous spectrum. 



(b) 4058-04. Lead? 



At normal atmospheric pressure this line is fine and sharp and apparently 

 superposed upon the less refrangible wing of line a (4055'44). From 5 to 25 



* Throughout this paper, unless expressly stated to the contrary, the pressure is the excess above 



1 atmosphere. 



