150 REPORTS OF INVESTIGATIONS AND PROJECTS. 



Spectra of Sun-spots. — Many photographs of spot spectra have been made 

 during the year by Mr. Adams and myself. Most of these have been taken in 

 the third order of the 30-foot spectrograph of the tower telescope and are 

 greatly superior to the photographs made with the Snow telescope and 18- 

 foot Littrow spectrograph. Double lines, which appear single in the previous 

 photographs, are now clearly resolved, and a great number of additional faint 

 lines, particularly those of flutings, are recorded. The preparation of a cata- 

 logue of the lines shown on the earlier plates was well advanced when the 

 first of the new photographs was obtained. Their superiority has made it 

 necessary to prepare a new catalogue, which will contain a much greater 

 number of lines than the former one. 



The hypothesis that the relative intensities of sun-spot lines are determined 

 largely by a reduction in the temperature of the metallic vapors below that of 

 the same vapors in the reversing layer is strongly supported by Dr. King's 

 work with an electric furnace, described on another page of this report. Dr. 

 Olmsted's detection of the red flutings of calcium hydride in spot spectra 

 affords additional evidence in the same direction. 



The discovery of the solar vortices suggested that the rapid revolution of 

 electrically charged particles (assuming a preponderance of positive or nega- 

 tive ions, resulting from diffusion or other cause) should produce a magnetic 

 field within sun-spots (Rowland effect). Photographs of spot spectra taken 

 with the 30- foot spectrograph and the tower telescope showed a great num- 

 ber of close double lines, many of which had previously been seen visually by 

 Young and Mitchell, who described them as "reversals." I accordingly ex- 

 amined the components of these lines and found their light to be circularly 

 polarized in opposite directions. This is what would be observed if the vapors 

 giving rise to these lines were seen along the lines of force of a strong mag- 

 netic field. Other spot lines, which are widened but not doubled, were found 

 to be shifted in position when the nicol was rotated. Thus the widening may 

 also be due to the effect of a magnetic field. If the laboratory work now in 

 progress supports the solar observations, this discovery should have an im- 

 portant bearing on many questions of solar, terrestrial, and molecular physics. 



Photographic Comparison of the Spectra of the Center and Limb of the 

 Sun. — The completion of the tower telescope, and the much more powerful 

 spectrographic apparatus available with this instrument, led to the transfer of 

 the spectroscopic work on various parts of the sun's disk from the Snow tele- 

 scope to the tower. The considerably greater dispersion available has proved 

 to be of particular value in connection with Mr. Adams's study of the spectra 

 of the center and limb of the sun, the quantities measured being so minute as 

 to require the greatest linear scale which it is possible to obtain. In order to 

 secure the two spectra simultaneously, and thus avoid possible errors arising 

 from change of temperature or unequal illumination of the grating in the 



