August 1, 1898.] 



KNOWLEDGE 



186 



below multiply and extend ; and just at the last moment 

 before totality is accomplished the continuous spectrum 

 is invaded by a number of dark longitudinal lines, resem- 

 bling the "dust lines " in an ordinary slit spectroscope. 

 The edge of the moon is of course somewhat rugged, and 

 here and there a mountain peak or range will project right 

 across the thread of sunlight which remains, and interrupt 

 the continuous spectrum at that point. But the effect 

 is not quite that of an ordinary " dust line," for if 

 the mountain, as at G, cuts out the sunlight, it does 

 not cut out the gaseous envelope above. This is free, 

 therefore, to yield its own bright-line spectrum, and 

 consequently we see our " dust line " sparkling out here 

 and there into stars of coloured light. H is an instance 

 where a lunar mountain hides the lower gaseous spectrum, 

 and allows us only to see the upper. P, again, is the 

 summit of a prominence which appears quite detached 

 from the sun, since its base is hidden by the moon. It 



— a most wondrous and beautiful sight, be its explanation 

 what it may. 



The " flash " was first seen by Prof. C. A. Young in 

 the eclipse of 1870, with a slit spectroscope attached to a 

 telescope, the slit being placed as a tangent to the limb of 

 the sun at the point of second contact. As he watched he 

 saw the ordinary solar spectrum gradually fade away, and 

 " all at once, as suddenly as a bursting rocket shoots out its 

 stars, the whole field of view was filled with bright lines." 



It win be seen, then, that this " bursting rocket " is but a 

 stage in a process that has been going on for some time. 

 It began at an early stage in the eclipse with the appear- 

 ance at the cusps of the bright arcs of H and K, of hydrogen, 

 and of helium, and these arcs have been multiplying right 

 up to the moment of totality ; but the final outburst is so 

 instantaneous, and brings so great a number of lines into 

 view, that it seems to stand out Uke a new phase in the 

 phenomenon. To vary the image, hydrogen, helium, and 



Fig. 1. — Diagram to illustrate the appearance and meaning of the Spectrum of an Eclipse, just before totality, as obtained with 

 a Prismatic Camera. Tlie left-hand figure represents the portiou of the Sun and its Atmosphere still visible b';yond the black disc 

 of the advancins Moon ; the right-hand figure the corresponding Spectrum. Lines a and d are supposed to be lines common to the 

 Sun, the " Flash," and the Chromosphere ; lines h and c are common to the Sun and the " Flash " ; line e is seen only in the Solar 

 Spectrum as a dark line. 



therefore shows itself in the spectrum by a row of tiny 

 colovired images of itself, shining like stars, quite detached 

 from the remainder of the spectrum. In most prominences 

 these will be the lines of hydrogen, helium, and the 

 celebrated H and K lines. 



The crisis is at hand ; the interruptions, which I have 

 likened to "dust lines," multiply and broaden. The inter- 

 vening continuous spectra are worn down to thinnest 

 threads, then snap and vanish, and totality has come. 

 The tiny stars which broke up the " dust lines ' flash out 

 as a long sequence of little arcs of colour, and shine for a 

 second, or perhaps two, ere the encroaching dark limb of 

 the moon covers the stratum to which they belong and 

 hides them from us. That brief, brilliant glimpse of little 

 bright-line arcs is what is known as the "flash" — the 

 " so-caUed flash," as certain over-cautious writers have 

 termed it, in the spirit of him who censured the manners 

 of this "so-called nineteenth century." " Flash " it is 



calcium might be likened to the three performers in a trio, 

 and now, as at the descent of the conductor's baton, they 

 have been joined by the complete chorus. 



The "flash," then, represents a shallow stratum of 

 glowing gases immediately surrounding the sun. The 

 height to which any particular gas can be traced can be 

 determined in three ways. First, by the length of the 

 bright-line arc beyond the cusp which it shows at any 

 particular moment ; nest, by the length of time that the 

 moon takes to hide the stratum ; third, by the extent to 

 which a given lunar mountain may interrupt the lines of 

 the gas at a particular moment. In one way or another we 

 find that, roughly speaking, the " flash" corresponds to a 

 stratum of some seven hundred miles in depth. 



When Prof. Young first saw the " flash " he considered 

 that the bright lines seen by him corresponded with the 

 ordinary Fraunhofer lines, and he remarks that though 

 " it would be very rash, on the strength of such a glimpse. 



