NA TURE 



501 



THURSDAY, APRIL 25, 1878 



THE COMING TOTAL SOLAR ECLIPSE > 

 II. 



IN my former article I referred to the possible employ- 

 ment of slitless spectroscopes during the coming 

 eclipse, the prism being replaced by a grating in some 

 cases. It will be convenient here to give the results 

 arrived at by the Siam Expedition with an instrument 

 of this description, which, for shortness, was called a 

 prismatic camera. 



The plates secured present at first sight a very puzzling 

 appearance ; they are unlike anything ever obtained 

 before, and a good deal of thought had to be spent upon 

 them before all the knowledge they were afterwards found 

 capable of furnishing to us was properly appreciated. 

 One of the plates was exposed for one minute at the 

 commencement of totality, the other for two minutes at 

 the end. The differences between them are those due to 

 the phases of the eclipse. In the first, two strong protuber- 

 ances close together are photographed ; these are partially 

 covered up in the second, while another series is revealed 

 on the following limb in consequence of the motion of the 

 moon over the sun. 



Now in both the photographs — that exposed for one 

 minute and that exposed for two— the strongest of the 

 prominences are repeated three times, that is to say, three 

 spectral images of them are visible, each of these images 

 being produced by light of different wave-lengths which 

 the prominences emitted. 



The question is what are these particular wave-lengths 

 thus rendered visible ? Unfortunately no photograph was 

 taken of the cusps either before or after totality; a scale 

 therefore was out of the question ; and when the task 

 of assigning wave-lengths to these spectral images fell 

 upon Dr. Schuster and myself, while we were preparing 

 the Report which was sent in to the Royal Society last 

 year, the difficulties we encountered were very consider- 

 able. 



Everybody I think will consider that we were justified 

 in expecting the lines of hydrogen to be represented in 

 such a photograph. Now the photographic hydrogen 

 lines are those at F, near G and at h, and the silver salts 

 usually employed are such that the action is most intense 

 near G, less intense near h, and least at F ; the running 

 down from G to F being rapid, and that from G to ^ 

 much more gradual, so that while at one end F may be 

 said to be the limit of photographic activity, at the other 

 it is continued long past h. We were therefore justified 

 in assuming as the preliminary hypothesis, that the image 

 of least refrangibility was produced by the F light of 

 hydrogen, the more so as the continuous spectrum also 

 photographed — which continuous spectrum, as we had 

 independent means of determining, came from the base 

 of the corona — gave us also an idea of the part of the 

 spectrum in which each image was located. 



Taking then F as a starting point and assuming the 

 next line to be the one near G, we had a quite satis- 

 factory method of checking the assumption, by com- 

 paring the real distance between the images with the 

 calculated one. 



' Continued from p. 483. 



Vol. XVII.— Nc. 443 



A goniometer was therefore brought into /equisition, 

 and the angular distance between F and the line near 

 G carefully measured in order to determine the dispersion 

 of the prism actually employed. This dispersion was one 

 which should bring the images about as far apart as they 

 were actually found to be ; this therefore was so far in 

 favour of our assumption, that is to say, it did look as if 

 we had got hold, on the photographs, of images of the 

 prominences built up by the F and G light of hydrogen. 



It was next the turn of the third line, the one at ^ Cn 

 the assumption already made, it was easy to determine 

 the distance from the G image, at which the one repre- 

 senting h should lie. In this place, however, we found 

 no image whatever of any of the prominences. 



Now this was a very extraordinary result, and there was 

 only one way, so far as we could then see, of accounting for 

 it. Dr. Frankland and myself, nearly ten years ago now, 

 produced evidence which seemed to indicate that this line 

 of hydrogen was only produced by a very high temperature. 

 This being so, then, we should have to conclude that the 

 prominences were of a relatively low temperature ; this, 

 however, I am far from saying, and here there is im- 

 doubted work of the greatest value to be done at the next 

 eclipse, and I for one feel certain that our American 

 cousins will do it. 



I have not, however, yet referred to the strongest image 

 of all shown in the photographs. This lies a little further 

 from the central one than does the first on the other side 

 of it. Cn the assumption before stated its wave-length 

 lies somewhere near 3957. This number, of course, is only 

 an approximate one, but the region occupied by the line 

 was obviously so near the boundary of the visible spectrum, 

 that a long series of experiments, in which we called in the 

 aid of photography and fluorescence, was made in order to 

 determine whether an unrecorded hydrogen line existed in 

 that region. All I can say is that the point may be said to 

 be yet undetermined. It is quite true that in several vacuum 

 tubes which Dr. Schuster and myself employed, a strong 

 line more refrangible than H was seen, but then these 

 same tubes unfortunately showed us lines in the visible 

 spectrum, which beyond all doubt did not belong to 

 hydrogen. The elimination of impurities is such a delicate 

 matter, and one requiring such a large expenditure of 

 time, that our report was sent in leaving this point sub 

 Judice. We tried hydrogen at atmospheric pressure in 

 order to get such a predominance of the hydrogen vibra- 

 tions as to mask the impurities, but this did not serve 

 us, for the continuous spectrum was so bright in the 

 violet and ultra-violet as to render observations of lines 

 next to impossible. Cwing to many reasons. Dr. Schuster' s 

 absence from London being one of them, we hare not 

 been able to renew the search. 



The near coincidence of this spectral image with 

 the H-line leads us to ask the question whether Young's 

 beautiful woric in his mountain observatory might 

 not help us on this point. Young found the calcium 

 lines always reversed in the penumbra and near every 

 large spot. This important statement shows us that 

 calcium is one of the metallic vapours which is most 

 frequently ejected from below into the prominences ; it 

 is possible, therefore, that the prominences, the spectral 

 images of which were photographed, may have been due 



