EFFECT OF PRESSURE UPON ARC SPECTRA. NICKEL. 213 



Except in a few instances the agreement is ^ood. When there were differences 

 Mr. PEABSE made additional readings ; from these and from the measurements of the 

 displacements at other pressures it was usually possible to decide upon the more 

 probable value. 



Experience in the measurement of displacements under pressure clearly shows that 

 the personal equation of a computer is not a fixed quantity ; there is a tendency for a 

 novice to record values that are too high, and it is found by experience that an interval 

 may make a considerable difference in one's judgment of a set of displacements. For 

 instance, the writer measured some lines before and after a voyage of some months' 

 duration, and found marked differences in the readings, the second set being only 

 about 70 per cent, of the first. There was, however, excellent agreement between the 

 rate of displacement with wave-length for lines with the same type of intensity curves. 

 It is on this account that the writer does not wish to lay too great a stress upon the 

 absolute values of the displacements for any one metal. The accuracy of the relative 

 values for different metals depends also upon the shapes of the intensity curves of the 

 lines ; if these are similar they are more likely to be reliable. 



In measuring displacements it is very important that the photographs shall be 

 illuminated by a source of constant brilliance, and for this purpose measurements 

 made in artificial light are more constant than those made in daylight of variable 

 intensity. 



(2) Description of Table of Displacements (l t<> 110 Atmospheres}. Table III. gives 

 in thousandths of an Angstrom Unit the value of the displacement of each line at the 

 pressure stated at the top of eacli column. The first column contains a list of the 

 arbitrary numbers assigned to the lines, the second the wave-lengths of the lines 

 according to HASSELBERO. The displacements measured for various pressures follow 

 in successive columns. 



Reversed lines are indicated in the manner stated in Section 5, p. 209. 



That the displacement increases with the pressure is at once evident. 



The second half of the table contains the displacements per atmosphere in 

 thousandths of an Angstrom Unit, the readings being obtained by dividing 

 those in the first part of the table by the excess pressure above that of one 

 atmosphere. 



A column is devoted to the Mean Displacement per atmosphere in thousandths of 

 an Angstrom Unit, to which reference will be made later ; and the final columns 

 which contain the quotient obtained by dividing the Mean Displacement per 

 atmosphere respectively by the first power, square and cube of the wave-length of the 

 line, will also be the subject of subsequent discussion. 



VOL. ccxv. A. 2 F 



