212 



DR. W. GEOFFKEY DUFFIELD ON THE 



(4) The Displacement Curves. 



The values of the displacements given in Table II. have been plotted in Diagram 1, 

 in which abscissae represent the excess of pressure above 1 atmosphere, and ordinates 

 the increase in wave-length of the lines in thousandths of an Angstrom unit. 



Each line on the diagram represents the behaviour under pressure of one spectral 

 line, which may be identified by the letter attached to it. Small arrows indicate 

 those pressures at which measurements have been made. Dotted lines connect 

 isolated observations. 



1300 



I2OO 



5 1100 



E3 



2 1000 



So 



cjjj 900 



DIAGRAM I. 



Pb 



Pressures in atmospheres (excess above one atmosphere). 



(5) The Relation between the Pressure, Displacement, and Wave-length. 



(1) Non- Series Lines.*- The increase of the displacement with the pressure is 

 shown in Diagram 1, and for each non-series line is continuous and linear within the 

 accuracy of the measurements. At low pressures (as we have seen) the displacements 

 on different plates are too discordant to be plotted, they are generally greater than 

 they should be if the linear law is to hold good, but, for the reason already pointed 

 out, that at 5 and 10 atmospheres the broadening seems proportionately greater than 

 the displacement, these measurements are not considered so reliable as the ones under 



* Not belonging to the Principal, First or Second Subordinate Series. See p. 219. 



