﻿280 Lord Rayleigh on the 



the temperature o£ the emitting gas is not much higher than 

 that of the containing tube. 



As regards m. no question arises for the inert monatomic 

 gases. In the case of hydrogen Buisson and Fabry follow 

 Schonrock in taking the atom rather than the molecule as 

 the moving source, so that m=l ; and further they find that 

 this value suits not only the lines of the first spectrum of 

 hydrogen but equally those of the second spectrum whose 

 origin has sometimes been attributed to impurities or 

 aggregations. 



In the case of sodium, employed in a vacuum-tube, 

 Schonrock found a fair agreement with the observations 

 of Michelson, on the assumption that the atom is in 

 question. It may be worth while to make an estimate for 

 the D lines from soda in a Bunsen flame. Here m=23, 

 and we may perhaps take T at 2500. These data give 

 in (14) as the maximum number of bands 



X;A = 137,000. 



The number of bands actually seen is very dependent 

 upon the amount of soda present. By reducing this Fizeau 

 was able to count 50,000 bands, and it would seem that this 

 number cannot be much increased *, so that observation 

 falls very distinctly behind calculation j\ With a large 

 supply of soda the number of bands may drop to two or 

 three thousand, or even further. 



The second of the possible causes of loss of homogeneity 

 enumerated above, viz. rotation of the emitting centres, was 

 briefly discussed many years ago in a letter to Michelson J, 

 where it appeared that according to the views then widely 

 held this cause should be more potent than (i.). The trans- 

 verse vibrations emitted from a luminous source cannot be 

 uniform in all directions, and the effect perceived in a fixed 



* " Interference Bands and their Applications," Nature, vol. xlviii. 

 p. 212 (1893) ; Scientific Papers, vol. iv. p. 59. The parallel plate was a 

 layer of water superposed upon mercury. An enhanced illumination may 

 be obtained by substituting nitro-benzol for water, and the reflexions from 

 the mercury and oil may be balanced by staining the latter with aniline 

 blue. But a thin layer of nitro-benzol takes a surprisingly long time to 

 become level. 



t Smithells (Phil. Mag. xxxvii. p. 245, 1894) argues with much 

 force that the actually operative parts of the flame may be at a much 

 higher temperature (if the word may be admitted) "than is usually 

 supposed, but it would need an almost impossible allowance to meet 

 the discrepancy. The chemical questions involved are very obscure. 

 The coloration with soda appears to require the presence of oxygen 

 (Mitcherlich, Smithells). 



X Phil. Mag. vol. xxxiv. p. 407(1892) ; Scientific Papers, vol. iv. p. 15. 



