A "reversing layer," or stratum of mixed vapours, glowing, but at a lower temperature than that of the actual solar surface, was an integral part of Kirchhoff's theory of the production of the Fraunhofer lines. Here it was assumed that the missing rays were stopped, and here also it was assumed that the missing rays would be seen bright, could they be isolated from the overpowering splendour of their background. This isolation is effected by eclipses, with the result--beautifully confirmatory of theory--of _reversing_, or turning from dark to bright, the Fraunhofer spectrum. The completeness and precision of the reversal, however, could not be visually attested; and a quarter of a century elapsed before a successful "snap-shot" provided photographic evidence on the subject. It was taken at Novaya Zemlya by Mr. Shackleton, a member of the late Sir George Baden-Powell's expedition to observe the eclipse of August 9, 1896;[529] and similar records in abundance were secured during the Indian eclipse of January 22, 1898,[530] and the Spanish-American eclipse of May 28, 1900.[531] The result of their leisurely examination has been to verify the existence of a "reversing-layer," in the literal sense of the term. It is true that no single "flash" photograph is an inverted transcript of the Fraunhofer spectrum. The lines are, indeed, in each case--speaking broadly--the same; but their relative intensities are widely different. Yet this need occasion no surprise when we remember that the Fraunhofer spectrum integrates the absorption of multitudinous strata, various in density and composition, while only the upper section of the formation comes within view of the sensitive plates exposed at totalities, the low-lying vaporous beds being necessarily covered by the moon. The total depth of this glowing envelope may be estimated at 500 to 600 miles, and its normal state seems to be one of profound tranquillity, judging from the imperturbable aspect of the array of dark lines due to its sifting action upon light.