POLARIZATION OF SKYLIGHT. TIM polarization of blue skylight was known to Huygens, and is evident to any one who will look at the sky through a prism of Iceland spar. After a systeumtic investigation of the distribution of polarized light in different parts of the sky, Arago (1811) discovered that a certain portion or spot in the sky shows no polarization. This point varies with the altitude of the sun. Sub sequently similar points were discov/ered by Arago (1840) and Brewster (1S42). The maxi mum polarization occurs in a narrow zone 90 degrees distant from the sun. Similar but feeble polarization occurs in moonlit skies. There is no polarization in fog, cloud. or dense haze. As there is an evident connection between the polarization of skylight and the quantity of moisture, haze, or smoke in the atmosphere, there have been numerous investigations of this subject both from an observational and a theoret ical point of view.
The simplest and most accurate po larimeter for study ing this phenomenon was devised in 1872 by Prof. E. C. Pick ering, director of the Harvard College Observatory. Obser vations made with this apparatus seem to show that the re flection of the sun light from the sur face of the sea into the atmosphere pro duced a decidedly perceptible disturb ance of the polariza tion of skylight as compared with that observed front inte rior stations where such reflection from water surfaces was impossible. Observations of
skylight during recent total eclipses of the sun seem to have shown that the amount of polarized light increases during and just before and after totality. Such an effect be produced either by the increased condensation of aqueous vapor. owing to the cooling of the atmosphere within the shadow of the moon, or by the fact that in place of the ordinary sunlight we now have to 410 with the polarized light from the solar corolla.
The most acceptable theory as to the origin of the polarization is that started by the experi ments of Tyndall on the reflection of sunlight from delicate clouds of vapor and perfected by Lord Rayleigh, who showed that when light is transmitted through a cloud of particles smaller than the wave length of the light, the latter is scattered laterally and polarized in the plane of scattering. In 1899 Rayleigh submitted the additional demonstration that not merely the grosser particles of dust and aqueous vapor, but even the molecules of oxygen and nitrogen, and probably those of other gases, take part in the process of scattering and polarization. Consult United ,States Monthly Weather Review (Wash ington) for September, 1900, where will be found a summary of recent work in this field and a hod iography.