146 Royal Institution : — 



Of course it is necessary for our purpose to allow only the edge of 

 the sun to fall on the slit, leaving apparently a large portion of the 

 latter unoccupied. What is seen, therefore, is a very narrow band in 

 the field of view of the little telescope and a large space nearly dark, 

 as the dispersion of the instrument is so great that the atmospheric 

 light is almost entirely got rid of, for a reason you are already ac- 

 quainted with. 



Mr. Ladd will now show you on the screen what is seen when the 

 slit reaches a prominence. First a line in the red, very obvious and 

 brilliant, next a more delicate line in the yellow, then another in the 

 green, and two others in the violet ; all these lines, with the excep- 

 tion of the yellow line, are in the positions occupied by known 

 lines of hydrogen. 



As the height of these bright lines must vary with the height of 

 the prominences, and as the lines will only be visible where there is 

 any hydrogen to depict, it is obvious that the form of the prominences 

 may be determined by confining the attention to one line, and slowly 

 sweeping the slit over it. 



The first fruits then of this new method of working with an un- 

 eclipsed sun was to tell us the actual composition of the prominences, 

 and to enable us to determine their shapes and dimensions. 



For the next steps you must permit me to refer more particularly 

 to my own observations. 



When I was first able to obtain results in this country similar to 

 those previously obtained by M. Janssen, though unknown to us, my 

 instrument was incomplete ; when other adjustments had been added 

 by Mr. Browning, I found that at whatever part of the sun's edge I 

 looked I could not get rid of the newly discovered lines. They were 

 not so long as I had seen them previously ; but there they "were, not 

 to be extinguished, showing that for some 5000 miles in height all 

 round the sun there was an envelope of which the prominences were 

 but the higher waves. This envelope I named the " Chromosphere," 

 as it is the region in which all the variously coloured effects are seen 

 in total eclipses, and because I considered it of importance to distin- 

 guish between its discontinuous spectrum and the continuous one 

 of the photosphere. And now another fact came out. The bright 

 line F took the form of an arrow-head, the dark Fraunhofer line in 

 the ordinary spectrum forming the shaft, the corresponding chromo- 

 spheric line forming the head; it was broad close to the sun's edge, 

 and tapered off to a fine point, an appearance not observed in the 

 other lines. 



Nature is always full of surprises, and here was a surprise and a 

 magnificent help to further inquiry lurking in this line of hydrogen ! 

 MM. Pliicker and Hittorf had already recorded that, under certain 

 conditions, the green line of hydrogen widened out ; and it at once 

 struck me that the " arrow-head " was nothing but an indication of 

 this widening out as the sun was approached. 



I will now, then, for one moment leave the observatory work to 

 say a word on some results recently obtained by Dr. Frankland and 

 myself, in the researches on the radiation and absorption of hydrogen 



