408 PROCEEDINGS OF THE AMERICAN ACADEMY. 



a trifling effect — which facts explain the old observation of Spottis- 

 woode that ivy leaves polarize particularly well. Grass, trees, stones, 

 especially if wetted, all produce their effect, which, when sky polariza- 

 tion is cut off by white cloud, is generally a maximum in the vertical 

 plane. 



I have several times observed this terrestrial polarization carried up 

 by reflection into low-lying cloud as noted by Pickering (loc. cit.), or 

 even into near-by dense fog otherwise entirely neutral. A completely 

 cloudy sky is otherwise practically free of polarization, but in a partially 

 clear sky white cumuli commonly show some effects with the Savart 

 plate, and light cirri often give bands almost as strongly as the clear 

 sky. This may be due to the usually considerable height of cirri, 

 — quite enough to allow noticeable polarization to have origin below 

 them, — or to their letting through considerable polarized sky light 

 from above, — a phenomenon which I observed from the summit 

 station in the case of rather thin layers of cloud in which it was 

 immersed. 



One of the most striking features of the sky polarization observed 

 from Breezy Point was the extent to wb.'ch it appeared while originating 

 over short stretches of air. Mounts Kineo and Cushman, about three 

 miles distant and dark with a heavy growth of conifers, repeatedly 

 showed strong polarization effects from intervening haze, and at times 

 slopes within a mile brought out the bands, although less conspicuously. 

 On several occasions the polarization on Kineo and Cushman was sen- 

 sibly as considerable as on peaks at ten or fifteen miles distance. 

 Similarly, in the brief observations on the summit, the Green Mountains 

 and the almost effaced Adirondacks showed little if any more polari- 

 zation than the peaks in the same direction in the middle distance, 

 although the former were eighty to one hundred miles away and the 

 latter only twenty to forty miles. These results follow from the ex- 

 ponential relation between distance and apparent absorption, but show 

 clearly the magnitude of the effects due to comparatively short reaches 

 of air. 



At no time was I able to repeat the results obtained by Tyndall in 

 the apparent clearing up of the haze by observation through a crossed 

 Nicol. In this case the mountains remained dim, Nicol or no Nicol, 

 showing that the typical autumnal haze, often whitish blue near the 

 horizon, acts mainly by general obstruction and diffusely reflecting 

 a good deal of light, the polarized component being usually only 

 moderately strong. 



Haze in general is well known to be due simply to suspended par- 

 ticles of one sort or another, and haze which produces polarization, as 



