}; (59) 



(60) 



98 Dr. C. V. Burton on the Scattering and 



Again, in place of (40) we shall have 



[Type I.] X2 v ~ l= i 7rvv ~ Z sm 2y = 3p\ 3 sin 2y/16ir 2 

 [Type II.] % 2 v~ l = ^ttvv~ z sin 2y= v\ z sin 27/I677- 2 



and in place o£ (41), for the extinction-coefficient, 



[Type I.] Xi = ^ 7rvv ^ 2 sm2 7 = 3X. 2 z/ sin 2 7/47^ 



[Type II.] %i= ttvv' 2 sin 2 7= \ 2 v sin 2 7/47r j 



49. The two types of molecule which have been especially 

 considered, though far from exhausting the a priori possi- 

 bilities, may serve as examples ; and it should be possible to 

 decide experimentally whether type I. or type II. represents 

 the more nearly the properties of the actual molecules of any 

 given gas — or whether a type distinctly different from either 

 is indicated. The suggestions now offered in this connexion 

 relate only to the means of discriminating between molecules 

 of type I. and those of type II. The most obvious method 

 is to use plane-waves completely polarized as the primary 

 disturbance, and to observe the intensity and degree of 

 polarization of the diffuse secondary radiation emitted in 

 definite directions. The density of the vapour should in 

 these tests be small enough to make the tertiary etc. 

 radiation insignificant in comparison with the secondary ; 

 otherwise the distinctions to be observed would be partially 

 obliterated, and the whole problem would become more 

 complicated. From the fact that Wood * observed no trace 

 of polarization in the radiation emitted laterally by his 

 ' resonance lamp," it may be surmised that a considerable 

 proportion of the resonance radiation was of tertiary or 

 higher order. For it seems hardly possible to imagine a 

 molecule such that the secondary f radiation emitted per- 

 pendicularly to the original existing beam would not be 

 at least partially polarized. Indeed Wood, in the papers 

 referred to, has emphasized the prominence of the radia- 

 tion of higher orders. To secure an approximately pure 

 secondary radiation, it may be necessary to use mercury 

 vapour under as low a pressure as *001 mm. contained in 

 a vessel of much smaller dimensions than those hitherto 

 used. In the following two paragraphs it is assumed that 

 such precautions have been taken. 



* Phil. Mag. May 1912, p. 712. 



t What is referred to above as "secondaiy radiation" is the same as 

 Wood's " primary resonance radiation," the term "primary" being in 

 this paper applied to the original incident beam. 



