1870.] 



TJie Solar Eclipse of August last. 



35 



Fig. 3. 

 c e 



JN 



reversing the dark line, but blazing like a star near the horizon, 

 only with blue instead of red light. It remained for about two 

 minutes, disappearing, unfortunately, whilst 

 the observer was examining the sun's image 

 upon the graduated screen of the slit, in order 

 to fix its position. It is not known, there- 

 fore, whether it disappeared instantaneously or 

 gradually, b gives an idea of this appearance. 

 On returning to the eye-piece, Professor Young 

 saw what is represented at c. On the upper e 

 (more refrangible) edge of f there seemed to 

 hang a little black moat, making a barb, whose 

 point reached nearly to the faint iron line just 

 above f. As given on Angstrom's atlas, the 

 wave-length of f is 486*07, while that of 

 the iron line referred to is 485*92 (the units being million ths of a 

 millimetre). This shows an absolute change of 0*15 in the wave- 

 length, or a fraction of its whole amount, represented by the decimal 

 •00030, and would indicate an advancing velocity of about 55*5 

 miles per second in the mass of hydrogen whose absorption pro- 

 duced this barbed displacement. The barb continued visible for 

 about five minutes, gradually resolving itself into three small 

 lumps, one on the upper and two on the lower line, Fig. 1, cl. In 

 about ten minutes more the f line resumed its usual appearance. 



Whilst on the subject of the solar prominences it may not be 

 out of place to refer to some observations by Professor F. Zollner, 

 who has succeeded in observing them without an eclipse with 

 great sharpness and clearness. From the nature of the method the 

 same protuberance was simultaneously observed in three different 

 colours corresponding to the three homogeneous lines of its spec- 

 trum. There is, however, a material difference between the red 

 and blue image on the one hand, and the yellow on the other. 

 The latter is very intense only in close proximity to the edge of 

 the sun's disc, and in this respect corresponds to the other images ; 

 while the more delicate details disappear at a greater distance. 

 This difference does not seem to be caused by the greater bright- 

 ness of the spectrum in that region, but appears to depend on one 

 of the two following hypotheses for an explanation : — either that the 

 rays which give rise to the yellow image emanate from a gas 

 having a greater specific gravity than hydrogen, and therefore 

 existing at a lower level, or that the greater intensities of tempera- 

 ture and pressure nearer the surface of the sun cause hydrogen to 

 emit these rays. 



Professor Zollner 's paper, which will appear in the next number 

 of the journal of the Franklin Institute (for early proofs of which 

 the writer has to thank Professor Morton, the editor), is illustrated 



d 2 



