170 Arc and Spark Spectra of JSickel under Pressure. 



Abnormalities or discrepancies are exhibited by unreversed 

 lines. It will be observed that this result is exactly the 

 opposite of that reached by Duffield's analysis of the data, 

 and supported by the theory he gives concerning the part 

 played by the density and temperature gradients. It remains 

 for further investigation to decide whether this view is 

 correct. The whole question turns very largely upon the 

 reality of an abnormally large rate of displacement with 

 pressure in the nickel arc at relatively low pressures ; and 

 confirmation of Prof. Duffield's conclusion may be regarded 

 as one of the most pressing problems in this branch of 

 spectroscopy. 



The part played by the temperature and density gradients 

 in determining the position of the maximum of emission or 

 absorption in a spectrum line is a problem which is becoming 

 increasingly more prominent with every fresh advance in 

 this line of investigation. It must be confessed that we are 

 still far from a complete interpretation of all the manifold 

 phenomena which present themselves. All that we can 

 really be sure about at present is that the measured displace- 

 ment is made up of a component directly due to the increase 

 of pressure and probably proportional to it, together with a 

 component of uncertain magnitude which has its origin in 

 the changes in the structure of the line due to the alterations 

 of the conditions prevailing in the source of emission. To 

 what extent it may be possible eventually to disentangle the 

 two components one cannot at present say. Meanwhile, as 

 Dr. Royds * has recently remarked, the most valuable 

 method of attack appears to be that of the electric furnace, 

 in which the vapour density is likely to be influenced by 

 pressure to a much less extent than in the arc or spark. In 

 any case, the recognition of the existence of these phenomena 

 is of the first importance, not only in interpreting existing 

 results, but also in determining the most profitable directions 

 for future research. 



Kew Observatory, 

 November 1915. 



Kodaikanal Observatory Bulletin, No. xliii. 



