114 THE CHEMISTRY OF THE SUN. [CHAP. 



purpose of thus seeing the shape and details of the promi- 

 nences ; how far the slit may be opened depends upon the purity 

 of the sky at the time. 



In 1873, Mr. Seabroke and myself communicated a method 

 to the Eoyal Society/ which enabled us to observe the whole of 



FIG. 45. Diaphragm showing ammlus, the breadth of which may be varied to 

 suit the state of the air. 



the chromosphere at once by means of a ring slit, which was made 

 coincident with an image of the chromosphere itself, so that the 

 light from the sun itself was stopped out. This arrangement 

 will be readily understood on referring to the diagrams (Figs. 45 

 and 46). 



FIG. 46. The anmilus is viewed and brought to focus by looking through aper- 

 tures in the side of the tubes. A, sliding eye-tube of telescope ; B, tube 

 screwing into eye-tube ; c, tube sliding inside B, and carrying lens D and 

 diaphragm E ; r, lenses bringing image of diaphragm to a focus at the place 

 generally occupied by the slit of the spectroscope ; G, collimator of spectro- 

 scope. 



It afterwards came to our knowledge that Zollner had 

 conceived the same idea, unknown to us, but had rejected it ; 

 and that Professor Winlock had tried a similar arrangement. 2 



1 Proc. Roy. Soc. vol. xxi. p. 105. 



2 For a more complete account of the methods of viewing the forms of the 

 prominences, see Solar Physics, p. 578 et seq. 



