Regular Reflexion of Light by Gas Molecules. 103 



hole in an opaque screen placed before the lamp. By means 

 of a quartz lens an image of the hole is formed on a photo- 

 graphic plate, a parallel-sided quartz cell, successively empty 

 and filled with low-pressure mercury vapour, being inter- 

 posed close to the lens. The illuminated hole being small, 

 the image formed on the plate need be but little affected by 

 the diffuse radiation from the absorbing cell. Since the 

 radiation from the resonance lamp over the range of frequency 

 , p 2 =p 2 top 2 =p 2 + d(p 2 ) will be approximately proportional to 

 sin 2 7, it is easy to see that the mercurial absorption will 

 reduce the brightness of the image in the ratio ($2); whence 

 by arithmetical trial and error we can find K, that is 

 ^cosec^L (where L is the internal thickness of the cell), 

 proceeding as in § 55. 



60. From the form of the expression (62) it seems to be a 

 not unreasonable conclusion that, ichen resonance radiation, 

 due to a single resonant frequency, is emitted from an atteu- 

 nuated vapour, the law of general enfeeblement of that radiation, 

 when passing through the same vapour, depends only on the 

 type and number per cubic centimetre of molecular resonators. 

 It is here to be understood that the resonance radiation is 

 emitted without either absorption or change of frequency, 

 and that the origiual excitation is due to a "lino'" much 

 wider spectroscopically than that which represents the re- 

 sonance radiation. The suggestion is put forward with some 

 reserve, because the relation (61) between 27rx frequency 

 (p) and phase-lag (7), involving also the dissipative co- 

 efficient k, is derived from the assumption of a formal 

 analogy between the resonating molecule and an ordinary 

 dynamical system. However, k disappears from (62), and 

 it is not easy to imagine the relation between frequency and 

 phase-lag so different in form from ((51) that the ratio (62) 

 would have to be replaced by one of a different order of 

 magnitude. The conclusion that only one molecule of 

 mercury vapour in several thousand forms an effective re- 

 sonator for X 2536 suggests the question : to how many 

 classes, differing spectroscopically from one another, do the 

 mercury molecules belong ? And in a vapour such as that 

 of iodine, which yields Aery complicated resonance spectra, 

 is the number of spectroscopic classes greater? Determina- 

 tions of the extinction of iodine-resonance-radiations by 

 iodine vapour, under conditions precluding true absorption, 

 might help to indicate how many effective resonators are 

 present for each line examined; but here fresh complications 

 arise. 



61. In the foregoing discussions, the aim has been to 



