0. A. Young on the Solar Corona. 315 



during eclipses, it i ^ 

 that the 1474 line ^ 

 troscopes.) 



If now we assume the chromosphere and leucosphere each to 

 be of uniform brightness throughout (a supposition not strictly 

 true but allowable for our present purpose), then by putting a 

 for the angular area of the chromosphere, x for that of the leu- 

 cosphere, i for the intensity of the bright 1474 line as seen in 

 an ** analyzing" spectroscope, and ni for the intensity of the 

 bright C line observed with the same instrument, we shall have 



*a:=[1474], 

 where [1474] represents the total quantity of " 1474 light" com- 

 ing from the whole leucosphere. Similarly 



n.i.a. = \G\ where [C] represents in the same manner the 

 total " C light" from the chromosphere. 



He.ee ....J^ 



Now, as nearly as I can estimate, the ratio between the briglit- 

 ness of the C and 1474 lines is about the same as that between 

 a star of the first magnitude and one of the fifth or si.xth. 

 That is to say, the C line is between twenty-five and one hun- 

 dred times as bright as 1474 ; so that in our equations we may 

 write n=:50 as a fair approximation, and we shall have 

 [14742 

 ^ [C] 



In order to find x in terms of a it only remains to find the 

 ratio of [1474] to [C]. For this purpose we must employ a 



spectroscope so arranged that its slit ' ' '"^ 



Jj,!*:^^ coming indiscriminately from 

 i when a spectroscope 



x-=ma^ 



ward the source of light. Thus used it mav be called an inte- 

 grating instrument, since it sums up in each bright line of its 

 spectrum, the whole quantity of each kind of light reaching tlie 

 sht from the whole luminous area included within its field. 

 . An instrument of this sort was employed by Prof Pickering 

 in 1869, and with results of considerable importance. 



Accordingly, two of the gentlemen of our party were er|U]p- 

 Ped in the saiie manner and one of them, Mr. Pye, by my 

 request, noted the brightness of the lines on an ai;bitrary 

 scale from ten down, with the following result: [CJ O'o, [D J 

 5'*^' [1474] 10, and [F] 3. Probably the squares of these num- 

 bers would more nearly express the true ratio of the inten- 

 sities, since we seem instinctively to estimate one light as 



