yO REPORTS ON INVESTIGATIONS AND PROJECTS. 



surface, and their total area is so great that the percentage error in 

 determining it should be less than in the case of sun-spots, in spite of the 

 fact that their boundaries are less sharply defined. I believe, however, 

 that the brightness of the flocculi, which varies greatly in different cases, 

 should be taken into account, as well as their area. For this reason I 

 have devised a photometric method of determining the total area and 

 brightness of the flocculi, in regions lo degrees square, and trust it will 

 prove suitable for routine investigations. 



Spectra of Sun-spots. 



In my last annual report, reference was made to the fact that photo- 

 graphs showing widened lines in the spectra of sun-spots had been 

 obtained, and that steps would be taken to interpret these phenomena 

 through experiments in the laboratory. The investigation which has 

 grown out of this work has been alluded to in the introduction to the 

 present report. The photographs of the spectra of sun-spots, made by 

 Mr. Adams and Mr. Ellerman, after the Littrow spectrograph of the 

 Snow telescope had been given its permanent form, proved so satisfactory 

 that they have served as the basis of our studies. It should be remem- 

 bered that the principal phenomena of spot spectra comprise: (i) Fraun- 

 hofer lines of the same intensity as those of the solar spectrum; (2) 

 lines that are widened or strengthened; (3) lines that are weakened or 

 replaced by bright lines. The photographs successfully register not only 

 the more conspicuous lines of these types, but also the multitude of fine 

 lines into which the spot spectrum was visually resolved by Professor 

 Young many years ago. In my photographic work on spot spectra at 

 the Yerkes Observatory, which was done wdth the assistance of Mr. 

 Ellerman, a few of the more conspicuous of these fine lines were recorded, 

 but the scale of the spectrum was so small that their identity could not 

 be determined with any certainty. The present photographs have a scale 

 five times as great, with the result that thousands of the fine lines appear 

 upon the plates, where their positions can be measured with precision. 

 Measurements of these lines, by Mr. Adams, have shown that most of 

 them are identical in position with the extremely faint lines recorded by 

 Rowland in the solar spectrum. In other words, this feature of the spot 

 spectrum is produced by a marked strengthening of the absorption of the 

 solar atmosphere. However, the conclusion drawn by Duner, from his 

 visual observations, "that there is no fundamental difference between the 

 general solar spectrum and that of the spots," is by no means warranted, 

 for the intensities of the lines which are strengthened are not increased, 

 in all cases, in the same proportion ; on the contrary, the changes show 

 the widest variations, some lines being unaffected, some enormously 

 increased in intensity, and others greath'- enfeebled. 



