ix.J VIEWING THE PROMINENCES. 113 



first published by Zollner, 1 in February, 1869, and adopted by 

 myself before I had heard of his paper. The method consists 

 simply in the use of a widely opened slit. Zollner, however, 

 fearing that the quantity of light admitted would be so great 

 as to obscure the forms of the prominences, proposed to reduce 

 it if necessary by polarising or absorbing media placed in the 

 eyepiece. 



On the 16th of the same month in which Zollner announced 

 his idea, Mr. Huggins 2 suggested a similar means of attaining 

 the same end by a combination of an open slit and ruby glass. 



On the 29th, I heard of this paper of Mr. Huggins', and it at 

 once struck ine that the absorptive media were useless, and that 

 the same result might be accomplished by the use of the open slit 

 only if the field was kept sufficiently small. On putting this to 

 the test I found my expectation fully confirmed. 8 



The way this method is applied will be readily under- 

 stood. 



Having found a prominence and got it carefully focused for 

 the c or F line, the slit is then simply opened widely, when 

 it is found that all the details of the prominence can be seen. 

 The reason of this will be clear on a little consideration. We 

 have already seen that the hydrogen Fraunhofer lines (like all the 

 others) appear dark because the light which would otherwise 

 paint an image of the slit in the place they occupy is absorbed ; 

 but when we have a prominence on the slit, there is light to 

 paint the slit ; and as in the case of any one of the hydrogen lines 

 we are working with light of one refrangibility only, on which 

 the prisms have no dispersive power, we may consider the prisms 

 abolished. Further, as we have the prominence image coincident 

 with the slit, we shall see it as we see the slit, and the wider we 

 open the slit the more of the prominence we shall see. We may 

 use either the red, or green, or blue light of hydrogen for the 



1 Aslronomische Nachrichtcn, No. 1772, September 15th, 1869. 



- Proc. Roy, Soc. vol. xvii. p. 302. 3 Ib. No. 110, 1869. 



I 



