16 Prof. A. W. Wrrarhl on the Polarization 



darkened field. The plane of polarization is easily determined 

 with it, since when the bright bands appear, as in fig. 1, the 

 longer diagonal of the Nicol is in that plane ; when the bands 

 are dark, the plane of polarization is parallel to the shorter 

 diagonal. 



On the completion of the instrument the first favourable op- 

 portunity was improved to test its efficiency upon the zodiacal 

 light. It was almost immediately found to indicate the exist- 

 ence of light polarized in a plane passing through the sun. 

 The bands were fainter than had been expected, and at first were 

 overlooked. More careful attention, however, and the obser- 

 vance of suitable precautions established their presence beyond 

 a doubt. The observations were made in a room in the upper 

 floor of one of the college buildings, the windows of which look 

 toward the south-west, and command a clear view nearly to the 

 horizon. The room during the observations received light only 

 from the sky, which sufficed to render objects dimly visible. After 

 being exposed only to this dim light for fifteen or twenty minutes, 

 the eye became sufficiently sensitive for observation. This was 

 a very necessary precaution, as a moment's exposure to a bright 

 light rendered the eye unfit for delicate discrimination of lumi- 

 nous intensities for a long time. The Nicol of the instrument 

 was now turned round and round, so that no previous know- 

 ledge of its position relatively to the bands of the quartz plate 

 might influence the judgment as to their character and position. 

 On looking through the tube at the zodiacal light, and turning 

 the whole instrument slowly round, it was possible to find a 

 position where the bands could be seen, and their nature and 

 direction determined. They could rarely be seen steadily by 

 direct vision, and then only for a few moments, as the excite- 

 ment and fatigue of the eye consequent upon the straining 

 effort of vision soon rendered the field a confused blur. Allow- 

 ing the eye to rest a few minutes, also on turning it obliquely 

 and rapidly directing it to different parts of the field, and espe- 

 cially by suddenly bringing it to focus upon the quartz plate, 

 the bands could be distinctly seen, and their direction fixed with 

 a good degree of certainty. On the clearest nights the brightest 

 bands (b, b, fig. 1) were seen without much difficulty, the broad 

 dark band (a), corresponding to an inclination of 45° in the 

 Nicol, less easily, and the dark bands (b, b, fig. 2) by glimpses. 

 After determining, by repeated observations, the angle made by 

 each of the bands with some fixed line, as the axis of the zodiacal 

 light, or a line nearly parallel to it drawn between two known 

 stars, the position of the plane of polarization was found, by 

 means of light from a gas- flame reflected from a sheet of white 

 paper placed in a suitable position, or by observing the position 



