of the Zodiacal Light, 1 7 



of the Nicol. The results of the numerous observations of dif- 

 ferent evenings were entirely concordant, and showed that the 

 plane of polarization passes through the sun, as nearly as it was 

 possible to fix its direction. In no instance when the sky was 

 clear enough to render the bands visible did their position, as 

 determined by the observations, fail to agree with what would 

 be required by polarization in a plane through the sun. Not 

 the slightest trace of bands was ever seen when the instrument 

 was directed to other portions of the sky. 



These observations, for the most part, were made in the ten 

 days preceding new moon in January and February of the pre- 

 sent year. During this time there was an unusual number of 

 clear nights, with the atmosphere cold and still. A few good 

 evenings in March and April also were improved in verifying 

 the results previously obtained. The absence of the moon, and 

 the distance of any of the brighter planets and stars from the 

 field of observation, removed all uncertainties from these sources. 

 As the instrument was directed to points from 30 to 40 or even 

 more degrees from the sun, the polarization could not have pro- 

 ceeded from faint vestiges of twilight. That it did not arise by 

 reflection of the zodiacal light itself in the atmosphere, or from 

 atmospheric impurities, is shown both by its amount and the fact 

 that it was always most easily discernible on the clearest nights. 



The next step was to determine what percentage of the light 

 is polarized. The failure of the common apparatus to detect it 

 shows that the proportion is not large ; but it must be recollected 

 that for a light so very faint much greater differences of inten- 

 sity are imperceptible than in cases where the luminous intensity 

 is greater. The determinations were made as follows. A bundle 

 of four pieces of excellent plate glass was placed vertically at the 

 centre of the horizontal divided circle of a DeleuiPs goniometer, 

 the telescope of which was replaced by the polariscope used in 

 the preceding observations. The latter was so placed that its 

 axis was perpendicular to the surface of the bundle when the 

 index of the goniometer was at zero. With the instrument thus 

 adjusted no bands are seen when unpolarized light is passed 

 through it ; but on turning the glass plates bands become visible 

 corresponding to polarization in a vertical plane. The amount 

 of the light polarized by refraction through four glass plates at 

 different incidences has been calculated by Professor W. Gr. 

 Adams* for intervals of 5° from 10° to 70°, and at 72°. Taking 

 the values given in his Table for crown glass (yu, = l , 5), those for 

 intermediate angles are readily determined by interpolation, or 

 graphically. The latter method was employed, a curve being 



* Monthly Notices of the Roval Astronomical Society, March 10, 18/1, 

 p. 162. 



Phil Mag. S. 4. Vol. 48. No. 315. July 1874. C 



