EFFECT OF PRESSURE UPON ARC SPECTRA. 



215 



Constants 



Maximum deviation , 



d ' 

 138 



per cent. 



4-3 



x A; 



d' d' 



608 268 



per cent. 

 4.4 



per cent. 



8-2 



In this case the best agreement appears when the displacement varies as the 5th or 

 6th power of the wave-length. 



(2) Series Lines. Table I. also gives the few measurements which were possible of 

 the lines belonging to the 1st and 2nd subordinate series. They are not sufficiently 

 numerous, nor is their accuracy sufficiently great, to determine the pressure- wave- 

 length-displacement relationships. 



(6) Mean Values of the Displacement. 



Taking the mean value of the displacements of these copper lines at each pressure, 

 the following numerical values are obtained : 



IO 20 30 40 SO 60 70 30 



Pressures in atmospheres (excess above one atmosphere). 



90 



IOO 



These values are plotted in Diagram 3. The mean displacement per atmosphere 

 of all copper lines fully examined is 12 - 2 thousandths of an Angstrom unit ; it is 

 probable that extension into the longer wave-lengths would increase and extension 



