BELL. — SOME METEOROLOGICAL USES OF THE POLARISCOPE. 409 



well as the ordinary sky polarization, is well known to be due to par- 

 ticles, whether of dust or water, or of other nature, small compared with 

 the wave-length of light. Lord Rayleigh 2 has given the theory of this 

 action in considerable detail. 



The polariscope integrates the effect of such particles along the line 

 of sight, and this information may have considerable meteorological sig- 

 nificance. The light-scattering particles which produce sky polariza- 

 tion are much finer than those which produce coronae and similar 

 phenomena, with the beginnings of ordinary reflection. In artificial 

 fogs the nuclei gradually grow from the polarizing dimensions to those 

 which scatter white light and become visible. It is not easy to assign 

 exact dimensions to the finer particles. They are quite certainly much 

 less than a quarter wave-length in diameter, that is, say 100 /*/a, and 

 probably run very much smaller. From the very exhaustive work of 

 Barus 3 it appears that the diameter of the particles to which visible 

 fog and coronae in a fog chamber of laboratory dimensions are due 

 range from .0005 /j. upwards, those near this limit showing as fog, while 

 the coronae began to form as the diameters reached 10 /a and above. 

 The fog particles to which lunar coronae are due often rise to greater 

 dimensions, 20 or 30 p. 



Now such fog particles are the preliminary to rain, which forms by 

 the accretion of these particles to a size that readily falls ; and it is 

 well known that water vapor, even when saturated as shown by the 

 psychrometer, will not begin to condense to visible fog unless in the 

 presence of nuclei about which aggregation takes place. These may 

 be of very fine dust , or even of water particles electrically charged to 

 an extent that resists the surface tension that would otherwise promote 

 evaporation. Such charged aqueous nuclei may exist in unsaturated 

 air at very small diameters, down to 1 or 2 /x/t, as has been shown by 

 J. J. Thomson, 4 by Wilson, 5 and by others. Between these almost mo- 

 lecular dimensions and those indicated by coronae are the light scat- 

 tering particles active in sky polarization. Their effect, that is, the 

 amount of light scattered, varies, as Rayleigh 6 has shown, as the inverse 

 fourth power of the wave-length of the light affected and directly as 

 their volume, assumed to be small compared with a wave-length. Now 



kA 



plotting the resulting equation, / = -j, one obtains a group of curves 

 shown in Fisrure 1 which discloses the cause of the familiar intense blue 



2 Phil. Mag., 1871, p. 107 et seq. 3 Smithsonian Cont., No. 1373. 



4 The Discharge of Electricity through Gases. 



5 Phil. Trans., 1897. 6 Rayleigh, loc. cit. 



