Arc and Spark Spectra of Nickel under Pressure. 165 



shown, further, that the displacement does not depend solely 

 on the applied pressure, but also on the density and tempe- 

 rature gradients, so that there is no a priori reason for 

 expecting strict agreement between the results for arc and 

 spark either in general or in any particular case. One must 

 confess, however, that the discrepancies actually found are 

 of a higher order of magnitude than might be anticipated. 

 Further work on the spark spectrum would undoubtedly be 

 desirable before making anything like a final decision. 



Personally, I should hesitate to draw any important con- 

 clusions from the fact that my values at five atmospheres 

 increase of pressure are relatively higher than those at ten 

 atmospheres. The smallness of the shifts renders the order 

 of accuracy rather low, and confers additional importance 

 upon the personal error and upon the various causes tending 

 to produce spurious displacements. Among these may be 

 mentioned the effects of granularity in the plates and of 

 lighting during the measurements. The spectrograph used 

 for my experiments w T as carefully designed to eradicate, as 

 far as possible, displacements due to changes of temperature 

 of the grating or mounting — a fertile source of trouble in 

 all work where the highest precision is essential. Even so, 

 it was found impossible to completely eliminate errors due 

 to this source, except, indeed, by increasing the number of 

 plates for measurement. With exposures of about the same 

 length, more uncertainty would be introduced in the cases 

 where the displacements are smallest, that is, at the lowest 

 pressures ; and this applies with equal force to all other 

 sources of error. With a large grating mounted in the 

 Rowland manner, as was employed by Duffield, the tempe- 

 rature effect would undoubtedly be very much larger. 

 Without labouring the point unduly, it will be realized that 

 we have here a cause which might easily account for the 

 whole of the observed discrepancies between Prof. Duffield's 

 displacements at ten atmospheres pressure and my own. 



Some further light is thrown upon the question bv referring 

 to the results obtained by Humphreys and Mohler* at pres- 

 sures of 9j, 12^, and 144 atmospheres, and by Humphrey- f 

 at the higher pressures of 42, 69, and 101 atmospheres. The 

 measurements at the lower pressures are somewhat meagre, 

 but show, on the whole, no large differences from the 

 results obtained by taking the means of the measurements 

 at all pressures. Thus the mean shift per atmosphere of 

 eight lines under pressures up to 14^ atmospheres is 2*0 



* Humphreys and Mohler, Astrophysical Journal, iii. p. 114. 



t Humphreys, Astrophysical Journal, xxvi. p. 36. 



