The "light" is usually seen after sunset or before sunrise. But attempts have recently been made by Prof. Simon Newcomb to observe it north of the sun. To avoid the effects of twilight the sun must be only slightly more than 18° below the horizon (that is, a little before or after the longest day). This condition limits the place of observation to latitudes not much south of 46°; and to reduce atmospheric absorption the observing station should be as high as possible above the level of the sea. Prof. Newcomb, observing from the Brienzer Rothorn in Switzerland (latitude 46° 47' N., longitude 8° 3' E.), succeeded in tracing the "light" to a distance of 35° north of the sun. It would seem, therefore, that the Zodiacal Light envelops the sun on all sides, but has a greater extension in the direction of the ecliptic.[258] From observations at the Lick Observatory, Mr. E. A. Fath found an extension of 46° north of the sun.[259] From observations of the "light" made by Prof. Barnard at the Yerkes Observatory during the summer of 1906, he finds that it extends to at least 65° north of the sun, a considerably higher value than that found by Prof. Newcomb.[260] The difference may perhaps be explained by actual variation of the meteoric matter producing the light. Prof. J. H. Poynting thinks that possibly the Zodiacal Light is due to the "dust of long dead comets."[261] From careful observations of the "light," Mr. Gavin J. Burns finds that its luminosity is "some 40 or 50 per cent. brighter than the background of the sky. Prof. Newcomb has made a precisely similar remark about the luminosity of the Milky Way, viz. that it is surprisingly small." This agrees with my own observations during many years. It is only on the finest and clearest nights that the Milky Way forms a conspicuous object in the night sky. And this only in the country. The lights of a city almost entirely obliterate it. Mr. Burns finds that the Zodiacal Light appears "to be of a yellowish tint; or if we call it white, then the Milky Way is comparatively of a bluish tint." During my residence in the Punjab the Zodiacal Light seemed to me constantly visible in the evening sky in the spring months. In the west of Ireland I have seen it nearly as bright as the brightest portions of the Milky Way visible in this country (February 20, 1890). The "meteoric theory" of the "light" seems to be the one now generally accepted by astronomers, and in this opinion I fully concur. From observations made in Jamaica in the years 1899 and 1901, Mr. Maxwell Hall arrived at the conclusion that "the Zodiacal Light is caused by reflection of sunlight from masses of meteoric matter still contained in the invariable plane, which may be considered the original plane of the solar system."[262] According to Humboldt, Cassini believed that the Zodiacal Light "consisted of innumerably small planetary bodies revolving round the sun."[263]