EFFECT OF PRESSURE UPON ARC SPECTRA. 



213 



higher pressures. The high value of the displacement may, perhaps, be referred to 

 the phenomenon discussed on p. 216, 8. 



The rates of increase of the displacements with the pressure vary greatly for 

 different lines. On the assumption that the displacement is a linear function of the 

 pressure, the displacement per 1 atmosphere has been calculated for each line between 

 pressures of 30 and 100 atmospheres, by dividing the measured displacement by the 

 number of atmospheres, and the results are given in the following table : 



TABLE III. Displacement per Atmosphere in Thousandths of an Angstrom Unit.* 



The last column gives the mean value of the displacement per atmosphere, and it is 

 at once apparent that these values increase with the wave-length. HUMPHREYS! has 

 suggested that these two quantities are dependent upon one another, and, though for 

 the investigation of this relationship the range of wave-length is small, the above 

 table supports that view. 



The precise nature of the relation between them is difficult to determine and may 

 be discussed from two different standpoints : 



(a) In Diagram 2 the average displacements per atmosphere are shown as 

 ordinates and the wave-lengths as abscissae. The graph which is obtained is approxi- 

 mately linear, and it also satisfies the requirements of line b due to lead ; it is seen to 

 point to about 3600 as the wave-length of the line which would not be displaced 

 under pressure ; but the work of HUM PHREYS, who has observed displacements of 

 lines with a smaller wave-length than 3600 A.U., indicates that this graph cannot 

 accurately represent the behaviour of the copper lines throughout the spectrum. 



* Measurements at 5, 10, 15 and 20 atmospheres are not included because their accuracy is not as great 

 as those made at higher pressures, nor are the readings for the faint lines /, h, k, which were measured by 

 dotting the film throughout the whole range. 



t HUMPHREYS, ' Astrophysical Journal,' VI., p. 169 (1897). 



