HUMPHREYS: DUST LAYERS AND POLARIZATION 241 



This is less than three times the amount of meteoric material 

 Young 7 assumes as allowable, and, so far as there is any present 

 means of knowing, may be even less than the actual amount 

 caught up by the earth per second. Indeed it is so small that 

 it would take about two hundred million years for it to increase 

 the radius of the earth a single centimeter! 



Numerical calculations, therefore, show that, though not 

 proved, it is within the bounds of reason to assume "earth light" 

 somehow due to bombardment of the outer atmosphere by fine 

 meteoric material; and hence the possible effect of such bom- 

 bardment should be taken into account in the planning of much 

 needed further observations. 



METEOROLOGY. — Dust layers in the atmosphere and changes 

 in the neutral points of sky polarization. W. J. Humphreys. 

 To appear in the Bulletin of the Mount Weather Obser- 

 vatory, 4: pt. 6. 



It is well known that sky light is, in general, partially polarized, 

 and that the percentage of polarization varies from one point 

 to another in the sky and also from day to day. 



If the light is analysed into vibrations parallel and perpendicu- 

 lar respectively to the horizon, certain neutral arches will appear 

 whose direction at every point makes an angle of 45° with the 

 plane of polarization. If however the analysis of the light is 

 not into these two arbitrary planes, but into the planes of maxi- 

 mum and minimum vibration, and if the direction indicated by 

 the maximum vibration be followed, one is led to a neutral point, 

 the lowest or highest, as the case may be, of the above mentioned 

 arches. The lowest, if it is above either the sun or the antisolar 

 point; the highest, if it is below either of them. In this latter 

 case however, only one of these points, the one below the sun, 

 known as Brewster's neutral point, can be actually observed, 

 the other is always in the shadow of the earth, and, in fact, is 

 purely imaginary. 



Along the sun's vertical, but between the sun and its nearly 



7 General Astronomy, p. 475. 



