20 On the Polarization of the Zodiacal Light. 



rough, a large number of substances possessing these charac- 

 teristics were subjected to examination with a polarimeter. For 

 this, purpose the apparatus already described was employed, 

 there being added to it a support for the object, with a hori- 

 zontal circle for determining the azimuths in placing the object 

 and the light. The substances examined had approximately 

 plane surfaces, which were placed vertically and so that the 

 normal, at the point observed, bisected the angle between the 

 lines from it to the eye and the illuminating flame. The light 

 being thus polarized in a horizontal plane, was depolarized (that 

 is, compensated) by turning the glass plates through the neces- 

 sary angle, the percentage corresponding to which was immedi- 

 ately found by means of the curve. 



If we suppose aline drawn from the place of observation to a 

 point in the zodiacal light, and another drawn from the sun to 

 this at its nearest point, the two lines would meet at right 

 angles; and a surface at the point of intersection must be so 

 placed as to have an incidence of 45° in order to send the re- 

 flected light to the eye of the observer. We may in general 

 assume that there would be as many meteoroids on the nearer 

 side of the line from the sun as on the other. Those on the 

 more remote side, while presenting a larger illuminated surface, 

 would reflect the light at a smaller angle, and therefore polarize 

 a smaller amount of it. Those on the earthward side would 

 send less light to the earth, but polarize a larger proportion of 

 it. The differences would so nearly complement one another 

 that we may take their united effect as equivalent to that of a 

 body placed at the point of intersection mentioned above. For 

 this reason the objects tested were so placed that the angles of 

 incidence and reflection were 45°. 



Some of the substances, and the percentages obtained, were 

 as follows : — Porphyry, ground smooth but not polished, 35 per 

 cent. ; another surface thickly covered with accumulated dust, 

 15*5; dark blue shale, 25*7; syenite, coarsely crystalline and 

 rough, 164; gneiss, rather fine-grained, 8*3; granite, fine- 

 grained, 11*8; red jasper, rough broken surface, 23'5 ; sand- 

 stone 12'1 ; brick, rough fragment, 8*1 ; the same, smooth sur- 

 face, 11*3 j red Wedgewood ware, unglazed, 14'2; indurated 

 clay, light brown, 11; mortar, whitewashed surface, 12'1 • the 

 same, rough side, 6 ; white chalk, cut plane, 2. A fragment of 

 the great meteorite of Pultusk, which the writer owes to the 

 kindness of Professor 0. C. Marsh, gave from a broken surface 

 11*7, from the blackened surface, 36 per cent, of polarized light. 

 It is of the stony class, and of a light bluish grey colour. 



The results show that from surfaces of this nature the light 

 reflected has in general but a low degree of polarization, not 



