170 



DE. T. E. MEETON AND PEOF. J. W.- NICHOLSON ON 



TABLE XIX. -General Emission of Helium Series. 



It is perhaps desirable to remark again at this point that the numbers in any one 

 column give no indication of the relative emissions of energy in these wave-lengths, 

 the properties of the plate being widely different in the three regions of the spectrum 

 concerned. Comparison is justifiable in rows, but not in columns. 



Low pressure produces a considerable enhancement of the Sharp series relatively 

 to the Diffuse series in fact the intensity is doubled and a simultaneous reduction 

 of the Principal series of an equivalent magnitude. 



A trace of Hydrogen produces only small differences in the relative emission of the 

 three series, as determined by these lines. The Sharp series is slightly enhanced, and 

 the Principal series to a somewhat greater degree in relation to the Diffuse series. 

 This enhancement of the Principal series continues, but not in a very important 

 manner, with the addition of more Hydrogen, whose effect, however, is very striking 

 in the case of the Sharp series, which is increased threefold in intensity, as compared 

 with the Diffuse. 



The corresponding table (Table XX.) for Parhelium is as follows: 



TABLE XX.- General Emission in Parhelium Series. 



The enhancement of the Sharp series found in the case of Helium, under low 

 pressure, may occur here also even to a somewhat greater extent, without appearing 

 on the plate, and the effect cannot be tested. But the most significant feature is 

 undoubtedly the behaviour of the Principal series characterised by \5015, which is 

 greatly enhanced in relation to the Diffuse series, in complete contrast to the 

 corresponding effect in Helium. 



