1869.] 



Observations of the Sun. 



353 



periodical elevations and depressions, and that at the epoch of minimum 

 sun-spot-frequency the plane might be uplifted very high in the solar 

 atmosphere, so that there was comparatively little cold absorbing atmosphere 

 above it, and therefore great difficulty in forming a spot. 



This suggestion is one of great value ; and, as I pointed out in my previous 

 paper, its accuracy can fortunately now be tested. It may happen, how- 

 ever, that in similar periodical fluctuations the chromosphere may be 

 carried up and down with the photosphere ; and I have already evidence 

 that possibly such a state of things may have occurred since 1860, for I 

 do not find the C and F Framihofer lines of the same relative thickness as 

 they were in that year*. I am waiting to make observations with the large 

 Steinheil spectroscope before I consider this question settled. But the 

 well-known great thickness of the F line in Sirius and other stars will point 

 out the excessive importance of such observations as a method of ascertain- 

 ing not only the physical constitution, but the actual pressures of the 

 outer limits of stellar atmospheres, and of the same atmosphere at different 

 epochs. And when other spectra have been studied as we have now studied 

 hydrogen, additional means of continuing similar researches will be at our 

 command ; indeed a somewhat careful examination of the spectra of the 

 different classes of stars, as defined by Father Secchi, leads me to believe 

 that several broad conclusions are not far to seek ; and I hope soon to lay 

 them before the Royal Society. 



For some time past I have been engaged in endeavouring to obtain a 

 sight of the prominences, by using a very rapidly oscillating slit ; but 

 although I believe this method will eventually succeed, the spectroscope I 

 employ does not allow me to apply it under sufficiently good conditions, 

 and I am not at present satisfied with the results I have obtained. 



Hearing, however, from Mr. De La Rue, on February 27th, that Mr. 

 Huggins had succeeded in anticipating me by using absorbing media and a 

 wide slit (the description forwarded to me is short and vague), it imme- 

 diately struck me, as possibly it has struck Mr. Huggins, that the wide slit 

 is quite sufficient without any absorptive media ; and during the last few 

 days I have been perfectly enchanted with the sight which my spectro- 

 scope has revealed to me. The solar and atmospheric spectra being 

 hidden, and the image of the wide slit alone being visible, the telescope or 

 slit is moved slowly, and the strange shadow-forms flit past. Here one is 

 reminded, by the fleecy, infinitely delicate cloud-films, of an English hedge- 

 row with luxuriant elms ; here of a densely intertwined tropical forest, the 

 intimately interwoven branches threading in all directions, the prominences 

 generally expanding as they mount upwards, and changing slowly, indeed 

 almost imperceptibly. By this method the smallest details of the pro- 



* I have learnt, after handing this paper in to the Koyal Society, that in Angstrom's 

 Map the C and F lines are nearly of the same breadth : this I had gathered from obser- 

 vations made with my own spectroscope. 



