of the Zodiacal Light. 1 5 



appearance of colour. Placed between two Nicols, the plate has 

 the appearance represented in the accompanying figures, which 

 are drawn of full size. When the corresponding diagonals of 



Fig. 1. 



the Nicols are parallel, or nearly so, the bands are white upon 

 a deep reddish-purple ground, as shown in fig. 1 ; with the 

 Nicols crossed, the bands are dark upon a light greenish-yellow 

 background, as represented in fig. 2. Turning one of the 

 Nicols 45° in one direction, the observer sees the central band 

 a intensely blue upon a yellow ground ; turning in the other 

 direction, a bright yellow upon a dark blue ; and intermediate 

 positions give the usual varying tints. Examined with one Nicol 

 and unpolarized light the plate is perfectly colourless, and shows 

 no trace of its heterogeneous structure. 



The quartz plate was placed in one end of a tube, large enough 

 to admit its full size very nearly, and 11 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 distinct 

 vision. In the other end was placed a good-sized 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 po- 

 lariscope of extraordinary sensibility, with faint light far excel- 

 ling the best Savart, and even with strong light somewhat 

 superior to it. The instrument is especially suited for the detec- 

 tion of small degrees of polarization, and the examination of 

 very faint lights. The occurrence of the narrow strips is pecu- 

 liarly advantageous, as with very feeble illumination they appear 

 bright upon a dark ground, or the reverse, and are thus more 

 easily seen. The efficiency of the instrument is further increased 

 by the comparatively large field of view and the perfect trans- 

 parency 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 appearing surprisingly luminous in contrast with the 



