104 POPULAR SCIENCE MONTHLY. 



of the spectrum lines. The writer obtained such photographs in 1898 

 and 1900, but with small instruments, not designed especially for that 

 work ; and it is hoped that improved apparatus will be available for the 

 eclipse of 1905. There is need that flash spectra with both fixed and 

 moving plates should be secured, since each system has its advantages 

 and disadvantages. On moving plates the faintest lines might not be 

 recorded, but a continuous record of changes in the strengths of lines, 

 as the moon gradually covers the reversing strata, should be obtained. 



The chromospheric stratum, overlying the photosphere, is of irreg- 

 ular depth, varying from four thousand to ten thousand miles. The 

 reversing layer, to the best of our knowledge, is included in its lower 

 strata. The prominences seem to be flame-like or explosive projections 

 extending outward from the chromosphere; the matter in them pre- 

 viously and subsequently forming a part of the chromosphere. Many 

 of the salient facts known about chromosphere and prominences were 

 learned at eclipses ; and they are still studied with some profit on such 

 occasions. However, the spectroscopic method of observing them, de- 

 vised independently by Janssen and Lockyer in 1868, has made the 

 prominences, and to some extent the chromosphere, available for every- 

 day study. But it must not be overlooked that, while fairly satis- 

 factory observations of one or both subjects can be secured without an 

 eclipse, yet the eclipse negatives are still imperatively needed to show 

 the mutual relations of the various structures — reversing layer, chro- 

 mosphere and inner gaseous corona. It is known that the prominences 

 are larger and more numerous at sunspot maxima than at other times. 

 The question whether the chromospheric stratum is likewise thicker 

 and more distorted at sunspot maxima than at minima is a question 

 for eclipse observers to settle. Observations of the continuous spec- 

 trum of prominences or chromosphere can by present methods be made 

 only at eclipses. 



The corona, perhaps the most fascinating solar feature, is exclu- 

 sively an eclipse phenomenon. Various attempts have been made to 

 observe it visually, photographically and thermally, without an eclipse ; 

 but all failed, and there seems to be no hope of success by methods now 

 known. Any chance for even moderate success would seem to be lim- 

 ited to the inner portion whose spectrum contains bright lines. A 

 daily record of this would, no doubt, be extremely valuable, but the 

 real problem of the corona would remain unsolved. 



In many respects the corona is as enigmatical as ever. A coronal 

 photograph is the result of a projection upon and into one plane, at 

 right angles to the line of sight, of all that remains of the sun after 

 subtracting the volume of matter hidden by the moon. The tops of 

 some coronal streamers, the intermediate portions of others, the bases 

 of those near the limb and the corresponding parts of prominences 



