XL] 



EVIDENCE AFFORDED BY THE SHIFTING OF LINES. 



105 



dispersion at Mr. Jewell's command will largely help 



enormous 

 matters. 



I now pass to Messrs. Humphre/s and Mohler's researches. 



These investigators used an electric arc enclosed in a cast-iron- 

 cylindrical vessel, which enabled them to vary the pressure up to four- 

 teen atmospheres. One hundred photographs of metallic spectra 

 were taken, and the shifts of some lines of twenty-three elements have 

 been measured. The accompanying rough diagram, bringing together 

 specimens of their observations, will indicate the kind of result they 

 have obtained. 



/O 



35. Changes of -ware-length produced by pressure, showing the different 

 behaviours of the lines of calcium (H and K and the blue line). 



The pressures in atmospheres are shown to the left. The shift 

 towards the red in thousandths of an Angstrom unit are shown below. 

 The shifts have been reduced to what they would be at A, 4000, in the 

 neighbourhood of which most of the work was done. 



The displacement or shift varied greatly for different elements. It 

 was always towards the red, and directly proportional to the wave- 

 length and the excess of pressure over one atmosphere. 



