July 31, 1919] 



NATURE 



427 



a new instrument having- a focal leng^th of 13 ft. 

 has also been employed. 



Fig. I is a typical photograph showing the 



the granulations of the photosphere which are 

 familiar to all observers of the sun (Proc. Nat. 

 .'Vcad. Sci., Washington, vol. ii., p. 95). The 



-Slit on centre of Ho, 5-ft. spectroheliograph, August 



Fig. i«. — Ordinary photograph of the sun, August 12, 1917. 



whole of the sun's disc when the second slit is 

 set on the centre of the Ha line. By comparison 

 with Fig. la, which is an ordinary photograph of 

 the sun taken at the same time, it will be seen 

 that, in addition to the granulation over the 

 general surface, there are both bright and dark 

 "flocculi" in the vicinity of the various spot 

 groups, and numerous dark markings in other 

 parts of the disc. It is now generally agreed that 

 both the bright and dark flocculi represent pro- 

 minences in projection on the disc, the bright 

 areas corresponding with bright eruptive promi- 

 nences, and the dark ones representing masses of 

 gas at the highest levels, where the temperature 

 is so reduced as to cause the hydrogen to show its 

 presence by absorption. This view has been 

 strongly supported by the stereoscopic examina- 

 tion of pairs of photographs taken at intervals of 

 about 7 mins., in which absorption markings have 

 appeared as high ridges. The long, dark flocculi, 

 which have been called "filaments" by Deslan- 

 dres, are therefore now regarded as long ranges 

 of prominences at a high elevation, but it is prob- 

 ably only the denser prominences which reveal 

 themselves in this way. 



The wealth of detail in such photographs can 

 only be properly appreciated by suitable magnifica- 

 tion of the original negatives, as in the other 

 examples reproduced. Fig. 2 is an interesting 

 case, showing several conspicuous prominences as 

 absorption markings, and the generally disturbed 

 state of the solar atmosphere in the region sur- 

 rounding a group of sun-spots. 



The small flocculi which appear all over the 

 sun's disc have been carefully studied by Prof. 

 Hale, who finds a general correspondence with 

 NO. 2596, VOL. 103] 



coarse "rice-grains" of the solar disc were re- 

 solved by Langley into clusters of minute and 

 intensely bright granules, not more than 0*3' 

 (about 135 miles) in diameter, and Langley re- 





Fk;. 2.— Slh on centre of Ho, 13-ft. spectroheliograph, Stptember 9, 1915. 



garded these as the upper extremities of long, 

 thin filaments which exist vertically aU over the 

 sun, and are revealed more completely in the 



