CAMPBELL— CROCKER ECLIPSE EXPEDITION. 251 



ence of observed wave-lengths is in the right direction. The chief 

 difficulty lies in the apparent fact that the green line is not strictly 

 monochromatic. Recalling the lumpy appearance of the green co- 

 ronal ring, we should perhaps be prepared for the probability that 

 there is motion within the green ring, such that the line as observed 

 by us is widened by radial velocity differences, and not reliably 

 measurable. 



The Wave-lengths of the Coronal Bright Lines. 



Much remains to be done in determining the accurate wave- 

 lengths of the coronal bright lines, in preparation for the chemical 

 identification of these lines. There are at least half a dozen coronal 

 lines whose position in the spectrum are determinable, with suitable 

 apparatus and care, much more accurately than they are now known. 

 Probably the best procedure is the unambitious one of attempting 

 to determine the position of only one line, or at most two neighbor 

 ing lines, with one instrument, exposing with as high dispersion as 

 good judgment dictates; covering with a narrow diaphragm the 

 region of the plate occupied by the coronal line while impressing 

 the appropriate arc or spark spectrum of an element, both shortly 

 before and shortly after the total phase of the eclipse. 



The Brightness of the Corona. 



The photometry of the corona is a problem worthy of further 

 attention, especially with reference to coordinating the laws of coro- 

 nal brightness with the sunspot phase. Studies in this field should 

 take into account the distribution of the coronal radiations through- 

 out the spectrum. The contrasting of the spectral photometry of 

 the outer, middle, and inner coronal structures is a most promising 

 problem, and the preparing of suitable apparatus, on the basis of 

 optical parts already existing in abundance, should be a simple 

 matter. 



The Flash Spectrum. 



The so-called flash spectrum of the sun's edge at the second and 

 third contacts should, in my opinion, be observed with instruments 

 specially and carefully designed, and adjusted with great care, to 



