166 Mr. E. G. Bilham on a Comparison of the 



thousandths of an Angstrom unit, compared with 1*8 from 

 the 42 atmospheres results and 1*9 from all measurements. 

 These lines belong to the types for which Duffield found the 

 greatest discrepancies between the arc and spark measure- 

 ments at ten atmospheres pressure — namely, those which are 

 easily reversed. There is, in general, good agreement 

 amongst the results of the two investigations of the arc. It 

 seems difficult to avoid the conclusion that for some reason 

 Duffield's measurements at ten atmospheres pressure are, on 

 the whole, too high, and, from what has been said with 

 regard to sources of uncertainty, the effect of temperature 

 changes suggests itself as the most likely cause. 



Comparison of Arc and Spark Displacements. 



In Table I. are given the mean displacements per atmo- 

 sphere in the arc found by Humphreys and Duffield 

 respectively, and the averages of the two sets of data, for 

 comparison with the spark displacements. Where only one 

 set of results is available, it is given in brackets under the 

 heading " Mean Displacement in Arc." Duffield's values at 

 ten atmospheres and Humphreys and Mohler's low-pressure 

 values are also given for reference. The class number for 

 each line according to its behaviour in the spark spectrum 

 under pressure is given in column 2. 



(Note :— 

 Class T. includes lines which reverse symmetrically. 

 Class II. ,, „ „ „ unsymmetrically. 



Class III. „ „ „ remain bright and fairly 



narrow. 



Class IV. „ „ „ remain bright, but are very 



much broadened sym- 

 metrically. 



Class Y. „ „ „ are very much broadened 



unsymmetrically to wards- 

 the red. 



The following classification exhibits a few alterations from, 

 that given in my original paper. After a careful re- 

 examination of all my photographs, the lines 3510*47, 

 3561*91, 3624-87, 3739*38, 3783-67, and 3831-87 have been 

 transferred from Class I. to Class II. The line 4359*76 has 

 been transferred from Class III. to Class V. 



Lines enhanced in the spark are indicated by the letter E.) 



