( 617 ) 



To this purpose the angles </^ for D and F have been reduced to 

 the same temperature by interpolation. 



Above 12°.54 the ratio of the intensities of D and F seems to 

 be constant. The fact already observed by several earlier observers 

 that on approach of the critical temperature the mist changes from 

 blue to almost white, is clearly set forth in the table. Measurements 

 on this change of colour, however, iiave been communicated here 

 for the first time. 



§ 4. On the size of the. Ug] it-scattering particles ^). To be able to 

 derive from rpiD the ratio of the intensities F and D of the light 

 scattered in a certain direction by the mist compared with the ratio of 

 the intensities F and D of the incident light on the mist, we must 

 bear in mind: 'J^*^ that the two beams of light which are compared 

 with each other in the spectroscope are subjected to different reflec- 

 tions and absorptions outside the spectroscope, which might bring 

 about a change in the ratio of the intensities D and F, 2"^ that the 

 optical apparatus for observation of the scattered light not being 

 perfectly achromatic might cause a similar change in the ratios of 

 intensities, 3''^ that if the condition of polarisation of the two beams 

 is not exactly the same on their arrival in the spectroscope, the 

 reflections in the spectroscope may also give rise to such a change "). 



The influences mentioned under 1 and 2 may be determined and 

 eliminated by measurements of the scattered light when the substance 

 in the neighbourhood of the critical state has been replaced by a 

 suspension for which the ratios of intensities of the scattered light 

 are known ^). Then it will have to appear in how far the deviation 

 of the values 2,00 found in table IV at the higher temperatures 

 from that which according to Rayleigh (Phil. Mag. (4) 41 (1871) 

 p. 107) would be found if the scattering were brought about by 

 non-conducting particles the dimensions of which are small with 



1) Gf. § 1 p. 612. 



2) Gf. Ghristie loc. cit. 



2) Suspensions for which the intensity of the transmitted light is: according to 



— JtA~*r 

 Rayleigh I= I^e : mastic, Ag Gl, Gu^ S in water, emulsion of lemon- 



essence in water : Abney and Festing, Proc. Roy. Soc. 40 (1886) p. 378, Lampa, 

 Wien. Sitz. ber. [2a] 100 (1891) p. 730, Hurion, G.R. 112 (1891) p. 1431, Gompan, 



G.R. 128 (1899) p. 1226; according to Glausius I =Ioe~^^ "" : Ba SO4 in a 

 mixture of glycerin and water, etc. : Gompan loc. cit. To ensure that in this expe- 

 riment the light is subjected to the same reflections as in the experiments with 

 the mist we should have to take a suspension in ethylene of the critical density. 



