342 SMITHSONIAN MISCELI^ANEIOUS COLI,e;CTIONS VOL. 52 



servers cooperating in the work of the International Solar Union, 

 this map has greatly facilitated visual observations, and has con- 

 siderably strengthened the view, now almost universally held, that 

 the Sun-spot spectrum undergoes few striking variations from spot 

 to spot or at different periods in the eleven-year cycle of solar ac- 

 tivity. 



The negatives having been secured and a preliminary map of the 

 spectrum prepared, it became necessary to draw up a catalogue of 

 all the lines affected, showing their intensities in the spot and in the 

 ordinary solar spectrum. The first section of this catalogue, extend- 

 ing from A 4000 (the extremity of the visible spectrum) to A 4500 in 

 the violet, has been published by Adams.^ In this limited region of the 

 spectrum, where the Sun-spot and solar spectrum were previously re- 

 garded as identical, about eight hundred lines of altered intensity 

 are recorded. The publication of the second section of the catalogue 

 has been somewhat delayed by the fact that negatives of the spot 

 spectrum made with the 30-foot spectrograph of the new "tower" 

 telescope (p. 356) are so much superior to the earlier plates that the 

 results obtained from them must also be added. As the complexity 

 of the spot spectrum increases from this region toward the green 

 and yellow, it is evident that the complete catalogue will comprise 

 many thousands of lines. 



Having thus acquired suitable data, the next step was to attempt 

 to interpret the true meaning of the Sun-spot spectrum. At this point 

 the need of laboratory experiments presents itself. Take, for ex- 

 ample, the spectrum of iron in a Sun-spot. The photographs show 

 that many of the iron lines are relatively much stronger than the cor- 

 responding ones in the solar spectrum, others are reduced in in- 

 tensity, and others are essentially unchanged. From experiments 

 on the spectrum of iron as observed in the laboratory, it is known 

 that the relative intensities of its lines depend upon the physical con- 

 ditions under which the vapor is observed — i. e., that variations in 

 the pressure, temperature, density, or electrical state of the vapor 

 are competent to affect their relative intensities. Adequate informa- 

 tion on this subject, however, is lacking. It was therefore necessary 

 to observe the effect of varying these physical conditions, in the 

 hope that the results might be applied to the interpretation of spot 

 phenomena. 



The apparatus provided on Mount Wilson for work of this char- 

 acter is illustrated in Plate xxix. Around the annular pier are ar- 



* Contributions from the Solar Observtitory, No. 22, Astrophysical Journal, 

 vol. 22, pp. 45-65, January, 1908. 



