EFFECT OF PRESSURE UPON ARC SPECTRA. GOLD. 



73 



Failing any practicable means of finding the above quantity experimentally, the 

 intensities have been gauged visually from the photographic plates by the most 

 intense portion of each line. 



TABLE IX. 



Caution must be exercised in comparing the intensities of lines upon the reproduced 

 photographs unless the lines under observation are very close together. In making 

 the prints, a small inequality in the illumination of two parts of the same plate may 

 cause very different photographic densities. In view of the great difficulty already 

 mentioned of estimating the intensities of lines under pressure, it is scarcely necessary 

 to add that the above table shows only changes in relative intensities. It was not 

 found feasible to fix upon an absolute standard for their comparison. 



To Prof. SCHUSTER and Prof. RUTHERFORD I express my thanks for their interest 

 in this research, and for having placed the necessary apparatus at my disposal. 

 I also thank Mr. R. Rossi, B.Sc., for his help in the early stages of this research. 

 Mr. W. C. LANTSBERRY efficiently assisted in the measurement of a large share of thi- 

 photographs. 



VOL. ccxi. A. 



