Mat 2, 1898.] 



KNOWLEDGE. 



113 



plantltory as embodying this idea ; and on page 382 the 

 footnote reads ; — " In Fig. '257 I indicate a way in which 

 the width of the penumbra on the side furthest from the 

 sun's edge (occasionally equal to the width of the side 



PHOTOGRAPHED 



Ideal Vertiual Secliou of a Sun spot in the earliest stage of its 

 deTelopment. 



nearest to the edge) may be explained by the refractive 

 action of the vapours within the spot cavity. The lines 

 Pp, Uuu, U'u'u', and P'p', are supposed to be lines of sight 

 from the earth when the spot is viewed aslant." 



We must, however, thank the Eev. A. East for the beauti- 

 ful experimental illustrations of this point. 



From the satisfactory way this theory simplifies many 

 of the difficulties connected with spot phenomena, it is 

 somewhat surprising that it has not found its way into 

 more general favour and recent text-books. 



Wm. Shackleton. 



Royal College of Science, London, 

 April 11th, 1898. 



SPECTRA OF 



: HERCULIS. 



o CETI AND 



By the kindness of the Kev. W. Sidgreaves we are enabled 

 to give the accompanying reproduction of two beautiful 

 photographs of these most interesting stellar spectra, ob- 

 tained at the Stonyhurst College Observatory in December 

 and February last. The spectra are on the same scale as 

 that of Ceti given in the number of Knowledge for March 

 (page 61), but are prolonged a little in both directions, 

 extending, in the orange, below the D lines ; and the 

 reference numbers to the great absorption bands, acciden- 

 tally displaced in the earlier reproduction, are here given 

 in their proper places. 



The juxtaposition of the two spectra brings out clearly 

 their very significant likenesses and differences. Under 

 the former head comes the series of great fluted absorption 

 bands which forms the distinguishing feature of this 

 stellar type. Under the latter may be noticed the greater 

 structural detail in the bands of a. Herculis, the presence 

 of numerous fine hnes, and the appearance, whether 

 actual or a mere contrast effect, as of a very bright line 

 to the blue of the sharp edge of the dark bands, especially 

 of bands four, five, and six. The spectrum of o Ceti, on 

 the other hand, is especially distinguished by its two bright 

 lines of hydrogen, y and i. The comparison of the two 

 spectra at the places of the three hydrogen lines renders 

 more distinct the relationship of these to the rest of the 

 spectrum, and emphasizes the remarkable fact of the 

 absence of the third line, H/3 ; the line which we should 

 have rather expected to show its presence with the greatest 

 plainness. — E.W.M. 



7 8 9 



(I) 



(2) 



Photographed Spectra of o Ceti (1), 1897, December, and o Herculis (2), 1898, February. 



Stonyhurst College ObserTatory. 



