The "secondary light" was also seen by Pastorff in 1822, and by Gruithuisen in 1825. Since 1824 observations of the "light" were made by Berry, Browning, Guthrie, Langdon, Noble, Prince, Webb, and others. Webb saw it with powers of 90 and 212 on a 9·38-inch mirror, and found it "equally visible when the bright crescent was hidden by a field bar."[42] Captain Noble's observation was rather unique. He found that the dark side was "always distinctly and positively _darker_ than the background upon which it is projected." The "light" was also seen by Lyman in America in 1867, and by Safarik at Prague. In 1871 the whole disc of Venus was seen by Professor Winnecke.[43] On the other hand, Winnecke stated that he only saw it twice in 24 years; and the great observers Dawes and Mädler never saw it at all![44] Various attempts have been made to explain the visibility--at times--of the "dark side" of Venus. The following may be mentioned[45]:--(1) Reflected earth-light, analogous to the dark side of the crescent moon. This explanation was advocated by Harding, Schröter, and others. But, although the earth is undoubtedly a bright object in the sky of Venus, the explanation is evidently quite inadequate. (2) Phosphorescence of the planet's atmosphere. This has been suggested by some observers. (3) Visibility by contrast, a theory advanced by the great French astronomer Arago. (4) Illumination of the planet's surface by an aurora borealis. This also seems rather inadequate, but would account for the light being sometimes visible and sometimes not. (5) Luminosity of the oceans--if there be any--on Venus. But this also seems inadequate. (6) A planetary surface glowing with intense heat. But this seems improbable. (7) The Kunstliche Feuer (artificial fire) of Gruithuisen, a very fanciful theory. Flammarion thinks that the visibility of the dark side may perhaps be explained by its projection on a somewhat lighter background, such as the zodiacal light, or an extended solar envelope.[46] It will be seen that none of these explanations are entirely satisfactory, and the phenomenon, if real, remains a sort of astronomical enigma. The fact that the "light" is visible on some occasions and not on others would render some of the explanations improbable or even inadmissible. But the condition of the earth's atmosphere at times might account for its invisibility on many occasions. A curious suggestion was made by Zöllner, namely, that if the secondary light of Venus could be observed with the spectroscope it would show bright lines! But such an observation would be one of extreme difficulty.