346 SMITHSONIAN MISCELI^ANEIOUS COLI.ECTIONS VOL. 52 



the complete interpretation of Sun-spot spectra, and do not even af- 

 ford conclusive evidence that reduced temperature is the principal 

 agent in determining the relative intensities of the lines. They 

 nevertheless carry us a step forward in our study of solar physics 

 and are of special service in illustrating the interdependence of solar, 

 laboratory, and stellar investigations. They render evident the im- 

 portance of increasing our knov^dedge of the Sun, of imitating solar 

 phenomena and interpreting solar observations by means of labora- 

 tory experiments, and of using these investigations as a guide to the 

 study of the stars and nebulae. 



Spi^ctra of tpie Limb and Center of the Sun 



Many years ago, when a student at Yale, Hastings made a com- 

 parative study of the spectra of different parts of the Sun's disk, 

 devoting special attention to any differences that might distinguish 

 the light of the center from that derived from points very near the 

 limb. Although his instruments were inadequate for the task and 

 his observations necessarily visual, he nevertheless noticed slight 

 differences in the appearance of a few lines. Strangely enough, the 

 importance of this work was overlooked by later investigators, though 

 Halm, two years ago, without perceiving the dift'erences noted by 

 Hastings, detected a slight displacement of certain lines at the limb 

 as compared with their positions at the center of the Sun. Halm's 

 work was also visual and accomplished with a comparatively small 

 spectroscope. Had he used a more powerful instrument and bene- 

 fited by the aid of photography, he would doubtless have discovered 

 the interesting series of phenomena which the Snow telescope and 

 18-foot spectrograph have brought to light. 



Some of these are illustrated in Fig. 2, Plate xxx, which represents 

 only one region of the spectrum. The broad diffuse wings which 

 accompany many lines are greatly reduced in intensity near the limb, 

 and in a number of cases disappear entirely. The relative intensities 

 of the lines themselves undergo marked changes, resembling in most 

 instances the changes observed in Sun-spots ; that is to say, the lines 

 that are strengthened in Sun-spots are usually strengthened near 

 the Sun's limb, while the lines that are weakened in Sun-spots are 

 weakened near the limb. However, the phenomena are by no means 

 strictly parallel, and much work will be required to arrive at their 

 true meaning. Perhaps the most interesting effects observed at the 

 limb are the displacements of the solar lines with respect to their 

 positions at the center of the Sun. In general, the relative displace- 



