184 



KNOWLEDGE 



[AususT 1, 1898. 



plain and free from spots, ignoring faculje which it is 

 unable to show. Meanwhile the spots may be there con- 

 cealed by faculcB, until some violent disturbance scatters 

 the faculaQ and the spot is suddenly revealed. 



Faculnc would be, according to the view here expressed, 

 identical with the photosphere, being only solar clouds at 

 a higher level, but distinct from the prominences, although 

 closely associated with them. 



[Kindly allow me to say that, when writing the article 

 on sunspots which appeared in the April Number of 

 Knowledge, I was quite unaware of the passage from the 

 late R. A. Proctor's " Old and New Astronomy," and of 

 his ideal section of a sunspot, quoted by Mr. Wm. 

 Shackleton, or I should certainly have referred to it ; but 

 I am grateful to Mr. Shackleton for drawing attention to 

 the passage. — Akthcr East.] 



THE OBJECTIVE PRISM, THE FLASH, AND 

 THE REVERSING LAYER. 



By E. Walter Maunder, i-.r.a.s. 



WITHIN the last few years a special form of 

 spectroscope has come largely into public 

 notice. Readers of Knowleuge need only 

 refer back to the number for March, 1897, and 

 they will there find, opposite page 78, six 

 beautiful photographs of stellar spectra. These, as Mr. 

 Fowler has fully explained in the article which accompanies 

 the plate, were taken with what is now usually called a 

 " prismatic camera " — that is to say, a photographic 

 telescope before the object glass of which a prism or train 

 of prisms had been placed. In effect this formed the 

 posterior half of a giant spectroscope, the anterior — the slit 

 and collimator — being absent. The slit was not needed, as 

 the star is itself but a point of light, as minute as the 

 narrowest slit ; the collimator was not necessary, as the 

 rays of light from the star were already parallel when they 

 reached the prism. The prism and view-telescope, there- 

 fore, were all that in this case were required. 



The spectrum of a star with such an instrument is a 

 very narrow line ; a broken line whose vacant spaces 

 represent the dark Fraunhofer lines that we see in the 

 solar spectrum. Such a broken line would be too narrow 

 for useful work, but by causing the telescope to move at 

 a slightly different rate from that of the star the latter 

 can be made to " trail," and thus the spcitrum may be 

 broadened out to any requireil extent. As will be seen by 

 reference to the plate in question, a star spectrum so 

 obtained looks exactly like the spectrum given by an 

 ordinary slit spectroscope. 



But this instrument is quite suitable for other kinds of 

 work, and its recent revival as an eclipse instrument, by 

 Sir Norman Lockyer, has shown it to be, taken all round, 

 our most powerful instrument for e<lipse research. 



But the appearance of the spectra of an echpse with a 

 prismatic camera is quite different from the spectra to 

 which we are ordinarily accustomed. If we look through 

 a prism at the young moon when she first sets her thinnest 

 silver crescent in the western sky, we shall see a spectrum 

 like that which an ordinary slit spectroscope will give us, 

 but with one great difference : the dark lines would no 

 longer be straight, but would be semicircles. All the 

 chief lines so familiar to us in the ordinary I'raunhofer 

 spectrum would be there, but instead of each being an 

 image in negative of a straight narrow slit, each is an 

 image in negative of the slender arch of the moon itself. 

 Exactly in the same way, if we watch through a prism the 

 coming on of an eclipse of the sun, we shall see, shortly 



before totality, when the encroaching dark disc of the moon 

 has reduced what is left of the sun to a thin crescent-like 

 arc, a spectrum with the Fraunhofer lines all circular arcs ; 

 images m negative, that is to say, of the little strip of sun 

 still uneclipsed, instead of the straight lines with which 

 the ordinary sht spectroscope has made us familiar. In 

 other words, we shall see spectra precisely like the first 

 and last of the splendid series of photographs which Mr. 

 Evershed gave us in the June Number. 



So far the matter is very plain, but just before second 

 contact the state of affairs becomes much more complicated. 

 It may appear a very obvious truism to say that, at any 

 moment during the eclipse, the spectrum which we obtain 

 is the spectrum of that bright object which is exposed to 

 our view at that moment, but it is a fact which has to be 

 very clearly kept in mind. In a slit spectroscope, the slit 

 is the source of light, for no other light is admitted to the 

 spectroscope except that which comes through the slit. 

 The slit may not be fully illuminated, and, in such a case, 

 it is only the lighted part of the slit which is the source of 

 our spectrum. But here, with a prismatic camera directed 

 towards an eclipse in progress, the source of light is the 

 whole of the phenomena — sun, chromosphere, prominences, 

 and corona — that at the moment of observation remain 

 uncovered by the dark body of the moon. 



The accompanying diagram (Fig. 1) may serve to show 

 just what it is which forms our source of light at the instant 

 before second contact. Let the arc C A B D represent the 

 dark approaching limb of the moon. The arc A K B 

 represents the limb of the sun, and, as we see, only a very 

 narrow segment of sunlight remains stUl disclosed. Be- 

 yond the sun's limb, however, there is a gaseous envelope 

 of which the chromosphere forms a part. For the sake of 

 distinctness I have supposed this envelope to consist of 

 two strata, an upper and a lower, and we may consider the 

 former as representing the chromosphere, the latter as 

 representing the " flash." 



What is the appearance of the spectrum at this moment ? 

 The small arc of sunlight still remaining gives us, of 

 course, a continuous spectrum, and it will be seen that 

 this continuous spectrum must narrow very fast as the 

 actual moment of totality comes on. This narrowing strip 

 of continuous spectrum is of course crossed by the Fraun- 

 hofer lines, each of which is of the same general shape as 

 the little arc of sunlight. But above and below this arc 

 of sunlight we find the dark limb of the moon bordered 

 only by the gaseous envelope. At the point of the cusps, 

 and a little beyond, we have both strata, but the lower 

 becomes narrower and narrower, and terminates at C and 

 D. The upper stratum can be traced further still, until 

 it, too, is cut off by the lunar limb at E and F. 



These arcs, then, AC, CE, and BD, DF, being arcs 

 simply of glowing gas, give us bright-Hne spectra. The 

 elements contained in each region will each give its own 

 spectrum of bright lines, and these bright lines will each 

 supply an image of the region over which that particular 

 element is found. Above and below the continuous 

 spectrum, therefore, with its arched Fraunhofer lines, we 

 find a bright-line spectrum of tapering horns of light of 

 different lengths, and we see at once that the length of any 

 j arc is an index of the height above the sun to which 

 that particular bright line can be traced.* 



As the fateful instant approaches, the continuous spec- 

 trum narrows faster and faster ; the bright horns above and 



* It does not, however, follow that the gas giving rise to the line 

 extends right from the sun's surface up to this height. It may simply 

 exist as a thin shell at the height thus determined. The different 

 effects in the two eases are not considered here. 



