NO. 1865 OUR KNOWLEDGE OF THE SUN HALE 34^ 



Other elements into simpler substances, whose spectra, being un- 

 known on the Earth, could not be identified. 



If we analyze the light of a Sun-spot with a spectroscope, we find 

 that the Fraunhofer lines of the solar spectrum are almost all pres- 

 ent, though their relative intensities are greatly changed. Many 

 solar lines, for example, are much strengthened or widened where 

 they cross the spot, while others are weakened or, in some cases, 

 completely obliterated. Lockyer's method of observation is to 

 record, day after day, the most conspicuous lines in the spot spec- 

 trum — those of the solar lines which are most widened or strength- 

 ened. Under the ordinary conditions of visual observation, the study 

 of the spot spectrum is a difficult operation, on account of the im- 

 mense number of lines afifected. Recognizing this, Lockyer confined 

 his attention to only twelve lines, in the expectation that their varia- 

 tions would sufficiently indicate the nature of any changes going on 

 within the spot. The inadequacy of this method has been shown by 

 recent results, which give no indication that the spot spectrum 

 undergoes a radical change in passing from maximum to minimum 

 solar activity, and demonstrate that an interpretation of the true 

 meaning of the strengthened and weakened lines must involve the 

 systematic study not merely of twelve lines, but of a far larger 

 number. 



When the Snow telescope was first employed for this work, only a 

 few hundreds of lines had been catalogued in the entire Sun-spot 

 spectrum. Previous experiments at the Kenwood and Yerkes ob- 

 servatories had indicated that the application of photography would 

 probably make possible an important advance, provided a spectro- 

 graph of sufficiently high dispersion were employed. A Littrow 

 spectrograph of i8 feet focal length, having a plane Rowland grating 

 ruled with 14,438 lines to the inch, was accordingly constructed for 

 use with the Snow telescope. Good photographs of spot spectra 

 were soon obtained with this instrument. After some minor tech- 

 nical difficulties had been overcome, it appeared that the photographs 

 could be counted upon to show nearly all that can be seen visually, 

 while at the same time they would permit the positions of the lines 

 to be accurately measured and their relative intensities to be deter- 

 mined. From negatives taken with the Snow telescope, Ellerman 

 prepared a preliminary map of the Sun-spot spectrum, extending 

 from the violet to the extreme red. Casual inspection of this map, 

 which comprises twenty-six sections of one hundred Angstroms 

 each, is sufficient to show that the number of lines whose intensities 

 are afTected in Sun-spots is several thousands. In the hands of ob- 



