A. W. Wright—Polarization of the Zodiacal Inght. 458 
marked appearance of color. Placed between two Nicols, the 
plate has the appearance represented in the accompanying fig- 
ures, which are drawn of full size. When the corresponding 
Bs 
diagonals of the Nicols are parallel, or nearly so, the bands are 
white upon a deep reddish purple ground, as shown in figure 1; 
q 
enough to admit its full size very nearly, and eleven inches in 
length. This was found better than a shorter one, as the bands 
are most easily seen when not nearer the eye than the limit of 
istinct vision. In the other end was placed a good size 
Nicol, and the tube was provided with a joint so that the latter 
could be easily turned. Thus mounted, the plate and Nicol 
form a polariscope of extraordinary sensibility, with faint light 
far excelling the best Savart, and even with strong light some- 
what superior to it. The instrument is especially suited for the 
detection of small degrees of polarization, and the examination 
of very faint lights. The occurrence of the narrow strips i 
peculiarly advantageous, as with very feeble illumination they 
appear bright upon a dark ground, or the reverse, and are th 
More easily seen. The efficiency of the instrument is further 
increased by the comparatively large field of view, and the 
Perfect transparency of the whole combination. 
As a test of its delicacy may be mentioned that when a glass 
Plate is laid upon the window-sill, and the light of the sky in a 
clear, moonless night, after reflection from it, is viewed through 
the instrument, both bright and dark bands are easily seen, 
the former a ring surprisingly luminous in contrast with the 
darkened field. The plane of polarization is easily determined 
With it, since when the bright nile appear, as in figure 1, the 
