10 



KNOWLEDGE 



[Januaby 1, 1898. 



give a slight general fogging of the plate. Evidently, 

 therefore, if we wish to truly sift out the light of the corona 

 from that of the prominences the latter instrument must be 

 employed. 



Although the prismatic camera has been used during 

 eclipses at various times since 1875, it was not until 1893 

 that sufficient dispersion and accurate focus were secured 

 in order to make use of the differentiation referred to 

 above, or it might be that the plates were not sensitive 

 enough to record the exceedingly delicate monochromatic 

 rings from the lower parts of the corona, which is the only 

 part left sufficiently bright after the great deduction that 

 must be made for the light giving only a continuous 

 spectrum. During the total eclipse of 1893 photographs 

 were taken in West Africa by Mr. A. Fowler, and in Brazil 

 by myself, which showed that the coronal light gave 

 rise to no H or K radiations of calcium — ^that the 

 prominences on the sun at that time had no 1474 K light : 

 and although this line, sometimes seen in eruptive 

 prominences, might be accounted for by supposing that it 

 really is the base of the corona which is being observed, or 

 that coronal matter has got entangled with the great 

 disturbances taking place, still in one such prominence 

 during that eclipse no trace of it could be found. Again, 

 in the eclipse of 1896, more than three years later, 

 the photographs show the same thing ; so we await with 

 interest the results of the coming eclipse, to see if in 

 passing from a maximum to a minimum sunspot period 

 any change takes place in the constitution of the corona. 



If a comparison be made of the K (calcium) and 1474 K 

 rings with a picture of the eclipsed sun, it is clearly 

 seen that 1474 K is truly coronal, and that H and K, 

 which are identical with each other, are solely due to the 

 prominences. 



Had these facts been sufficiently well established in 

 1893, M. Deslandres might not have tried in vain to 

 determine the rotation of the corona by photographing 

 the relative displacement of the II and K lines on opposite 

 limbs of the sun. In the last number of Knowledge it 

 was stated that Mr. Newall is going to try to make the 

 same observation, using a "bright line near (1, of whose 

 coronal nature there can be no doubt." Let us hope he has 

 consulted the records of the prismatic camera before doing 

 this, for although one such line was tabulated at ^ 4232-8 

 by Schuster in 1886 as being the brightest in the photo- 

 graphical region, the results of 1893 and 1896 show that 

 a bright line near H, A 3987, is more intense than this, and 

 in fact is the next strongest line to the coronal Une 

 (1474 K) itself. This will be seen on examining the 

 photograph taken near mid-totality, and reproduced here 

 in the plate by the kind permission of the Royal Society. 



So far we have examined the capabilities of the pris- 

 matic camera for giving us information about the parts 

 of the sun comparatively well removed from the photo- 

 sphere ; let us now turn our attention to see what can 

 be done with it for the investigation of those vapours 

 which lie closer in, in order to test Kirchoff's theory 

 " that the absorption which produces the dark Fraunhofer 

 lines takes place in a thin stratum, or reversing layer, as it 

 has often been called, adjacent to the photosphere." 



In a total eclipse of the sun, at the moment the 

 advancing moon just covers the sun's disc, the solar 

 atmosphere of course projects above the dark edge, and at 

 that moment the reversing layer will be isolated for only a 

 very few seconds. If, now, at this precise instant, a photo- 

 graph be taken with the prismatic camera, we shall have 

 the spectrum of this shallow layer, chromosphere and 

 corona ; but from the form of the arcs and their appearance 

 or non-appearance in later photographs, we shall be able 



to separate the integrated effect into its indindual parts. 

 From the very nature of this layer and the inequalities in 

 the moon's position, the difficulties in the way of making 

 the exceedingly fine adjustment of placing a ^/'> upon this 

 point of disappearance are almost insurmountable; in fact, 

 so great are they that it was not until the apphcation of 

 the prismatic camera, which requires no such nicety of 

 adjustment, that there was any permanent record of this 

 low-lying stratum. 



Except at an eclipse it has not yet been found possible 

 to observe this bright line spectrum, because it is over- 

 powered by the aerial illumination of our own atmosphere 

 so spectroscopists are the more anxious to make the most 

 of every echpse to settle at least this one point. With 

 this end in view many prismatic cameras have been 

 directed to the eclipsed sun, but it was not until 1893 

 that anything like the base of the sun's atmosphere was 

 photographed. 



The difficulties of placing a slit on a point have been 

 mentioned previously, but not only is there that to contend 

 with, but also, no matter what instrument be used, the 

 exposure must be made at the precise moment the sun's 

 disc is covered. To do this, Sir Norman Lockyer, during 

 the eclipse of 1896 in Norway, instituted a " running 

 plate," which took a series of snapshots just before and 

 going on till the critical moment had passed — in fact, a 

 sort of kinematograph arrangement. Unfortunately, how- 

 ever, the weather was unfavourable to let us see what 

 results this method would give. Mr. Evershed, also, in 

 clouded-out Norway, and myself, in Novaya Zemlya, rehed 

 more on the exact determination of the proper instant, 

 and then making a short exposure. What such a photo- 

 graph is Like, and how far it agrees with a reversed solar 

 spectrum, can be gathered from the plate, which is a repro- 

 duction of the Novaya Zemlya photograph. Of course 

 any comparison must be made with a spectrum obtained 

 by a similar instrument, for it would obviously be fallacious 

 to compare a spectrum taken with only a moderate-sized 

 spectroscope, making clear to us only a few hundreds of 

 lines, with such a spectrum as that taken with a Rowland 

 grating, which reveals in tin snine sunliiiht as many tens of 

 thousands of lines. Therefore, the only way of absolutely 

 proving that every fine dark line is reversed would be to 

 photograph this layer with a Eowland grating, which, with 

 our present appliances and the short duration of visibility, 

 is nearly impossible : but this is to be tried by Prof. Michie 

 Smith during the forthcoming eclipse. Still, notwith- 

 standing these difficulties, the investigation of the Imes 

 in the photograph is proceeding at the Solar Physics 

 Observatory, South Kensington; but, probably, before it 

 is finished we shall have many such photographs, with 

 more powerful instruments, from India, where the sun will 

 be bombarded, not by one only, but by at least half a 

 dozen prismatic cameras. 



With such possibilities in an eclipse, no wonder during 

 such times that the sun monopolises the attention of 

 astronomers, not only for the secrets he has to divulge of 

 himself, but also for the key he may possibly give to cipher 

 the constitution of other countless suns more remote. 



NOTES ON COMETS AND METEORS. 



By W. F. Dennujg, f.k.a.s. 



Comets. — 1897 has afl'orded only one new comet — that 

 discovered by Perrine on October 16th. When first seen, 

 the comet was placed in the south-east region of Camelo- 

 pardus and moving north-west ; it has since traversed 

 Cassiopeia, Cepheus, and Draco. Early in January, 



