January 1, 1894.] 



KNOWLEDGE. 



19 



almost identical in appearance, except that the latter is 

 always the brighter of the two. 



The question has been raised as to whether the bright 

 lines with dark centres are true double reversals, or simply 

 close double lines. A moment's consideration of the facts 

 of the case ought to leave no room for doubt on this point. 

 If a photograph of the spectrum is taken with the slit 

 lying across the sun's limb, it is found that the two bright 

 lines of the double reversal unite into a single bright line 

 at the edge of the disc, and this single bright line exactly 

 coincides in position with the central dark line of the 

 double reversal. This is shown in the photograph from 

 which the cut was made, but the reproduction fails to 

 bring it out. 



The investigations of C'ornu and other physicists, on the 

 reversal of the lines of metallic vapours in the electric arc, 

 offer most striking analogies to the phenomena just de- 

 scribed. Through the kindness of M. Cornu, I have 

 recently had the pleasure of examining some of his photo- 

 graphs of reversed lines. The investigations were confined 

 to the ultra-violet, and the calcium lines which correspond 

 with H and K were not specially studied. The reversals 

 of other lines are similar, however, and will serve our 

 present purpose equally well. An ultra-violet line of 

 aluminium on one of the photographs reproduces the solar 

 H and K reversals so perfectly that it might readily be 

 mistaken for one of them, were there any doubt as to its 

 mode of production. 



In M. Cornu's experiments an image of the arc was 

 formed on the slit of the spectroscope. A portion of 

 the metal, or one of its salts, was introduced into a 

 cup-shaped cavity in the lower carbon, and vaporized 

 by the passage of the current. Under these conditions 

 the central portion of the arc, where the line of sight 

 passed through the cool exterior vapour to the intensely 

 heated vapour and the glowing carbon poles, showed 

 the aluminium line reversed — two narrow bright lines 

 enclosing a narrow dark line. At the edge of the arc, 

 however, where the line of sight passed through only 

 the cooler vapour of the exterior, the two bright lines 

 united into a single bright line, corresponding in position 

 with the dark line seen in the first case. 



We thus arrive at a basis for the interpretation of the 

 H and K reversals in the sun. In the arc we have a 

 reversed line produced by the absorption of the cooler 

 vapour of the exterior. The chromosphere would seem 

 to play the part of the absorbing vapour in the sun. 

 At the base of the chromosphere, or below it, is the 

 hotter vapour corresponding with that at the centre of 

 the arc. Here is the seat of the brilliant radiation of 

 calcium, which produces the two bright components of 

 the doubly-reversed H and K lines. The upper part of the 

 chromosphere, on the other hand, acts as an absorbing 

 screen, and produces the central dark line of the reversal. 

 A photograph of the solar disc secured by means of the 

 spectro-heliograph (using the K line) should therefore 

 show, not the entire chromosphere, but only its lower and 

 hotter parts. 



The distinction is an important one when it is 

 remembered that photographs taken with the spectro- 

 heliograph show the entire surface of the sun to be 

 mottled over with small, irregularly-shaped, bright regions, 

 which seem to form a nearly unbroken reticulation 

 {Astronomy and Astro- Phi/sics, May, 1893, p. 450). This 

 is not to be mistaken for the well-kno-(\Ti "granulation,'' nor 

 for the "reseau photospherique " of M. Janssen. Neither 

 does it seem at all probable that the brighter regions are 

 merely elevations in the chromosphere, for if the upper 

 part of the chromosphere acts as an absorbing medium, 



and produces the dark central line of the H and K reversals, 

 an increase in the depth of the chromosphere would certainly 

 not diminish the absorption. It seems likely that the 

 reticulation represents a true facular network, for thtf 

 small facuLe seen without the spectroscope near the sun's 

 limb, and well described by Secchi {Le Soleil, German 

 edition), are probably parts of the same reticulation. 

 Probably no sharp distinction can be made between the 

 base of the chromosphere and the upper surface of the 

 underlying faculse. The latter seem to be in intimate 

 connection with the interior of the sun, and one might 

 therefore expect to find the H and K lines very bright in 

 them. 



In this connection it should be remarked that the H and 

 K lines over spots are frequently somewhat narrower and 

 less brilliant than on the disc, and the central dark line is 

 often absent. I have explained this as probably due to the 

 fact that we are here dealing with the radiation of the 

 chromosphere overlying the cooler region of the spot. 

 (Astrono?iri/ and Astro- Plnjsic^, 1892, p. 815). 



In a recent paper on the " Physical Constitution of the 

 Sun " {Astronoiiii/ and Astro-Physics, 1893, p. 832), Father 

 Sidgreaves has expressed his belief that faculfe are promi- 

 nences seen in projection on the solar disc, and 

 M. Deslandres has advocated a similar hypothesis in 

 the December number of Knowledge (see also Cowptcs 

 rendiis, Nov. 27th, 1893). As my own position in regard 

 to the subject is evidently not fully apprehended by M. 

 Deslandres, I shall endeavour in what follows to state it 

 as clearly as possible. 



In a note dated January 18th, 1892 (Astromomy and 

 Astro-Physics, February, 1892, p. 159), I wrote as follows 

 in regard to the regions on the sun's surface in which I 

 had found the H and K lines to be doubly reversed : " On 

 January 12th, 1892, it was found possible to photograph 

 the /'(irrns of some of these reversed regions, using a moving 

 slit apparatus just completed for our large diffraction 

 spectroscope by Brashear. The K line in the fourth order 

 spectrum was employed, as is customary in the case of 

 prominences. The reversed regions are of great extent, 

 and in appearance closely resemble faculfe. Several 

 explanations may be suggested to account for them. They 

 may be : — 



" 1. Ordinary prominences projected on the disc. 



" 2. Prominences in which H and K are bright, while 

 the hydrogen lines are absent. 



'' 3. Faculfe. 



"4. Phenomena of a new class, similar to faculie, but 

 showing only H and K bright, and not obtained in eye 

 observations or ordinary photographs because of the 

 brilliant background upon which they are projected." 



Subsequently I found, by comparing photographs of 

 faculie near the sun's limb, made at the focus of a telescope 

 in the ordinary manner, with photographs of the reversed 

 regions made with the spectro-heliograph, that there was a 

 very close agreement in form. For this reason I adopted 

 the provisional name " faculaj " in subsequent references 

 to the bright regions shown on spectro-heliograms, 

 reserving an exhaustive discussion of the phenomena until 

 sufficient material had been collected for that purpose. 

 Part of this material, in the form of about three tliousand 

 spectro-heliograms and several hundred ordinary photo- 

 graphs of the sun, has already been collected and partially 

 reduced. It is intended that one of tlie first volumes of 

 publications to be issued by the Yerkes Observatory shall 

 be devoted to a discussion of these results. At the present 

 time, and at a distance from my photographic and other 

 records, I can discuss the subject only in a provisional 

 way. 



