CH. XXIII.] 



METHOD ADOPTED. 



311 



The figure (Fig. 105) shows a limited part of the solar spectrum, 

 and the lines thickened in the spot-spectrum. It will be seen 

 therefore that to tabulate the existence, thickness and intensity 

 of these lines over the whole of the solar spectrum would be 

 a work which it would be difficult to accomplish in a single day 

 even if the day were absolutely fine. So that was given up in 

 favour of a limited inquiry over a small part of the solar spec- 

 trum ; limited further by this, that we only note the twelve lines 

 most affected in each spot on each day. In this way we insure 

 a considerable number of absolutely comparable observations 



FIG. 105. Part of the spectrum of a sun-spot observed at Greenwich. 



and we can more easily compare the spot results with those 

 which have been obtained in the observation of the brightest 

 lines in prominences; because when one begins to observe lines 

 in the solar prominences one naturally observes the brightest lines 

 first. So that by observing the darkest lines first in the case 

 of spots one has a fairer comparison. 



The work in question has now been going on uninterruptedly 

 since 1879. From the first I felt it was crucial, and that I was 

 in duty bound to proceed with it to see if it would supply us 

 wuth any additional tests. 



The work enables us to discuss the lines of several chemical 

 elements most widened in 700 spots observed at Kensington. 

 The period of observation commenced November, 1879, and ex- 

 tended to August, 1885. It includes, therefore, the sun-spot curve 

 from a minimum to a maximum and some distance beyond. 



