Jahtjart 31, 1908] 



SCIENCE 



167 



points at the center and near the limb 

 of the sun's disk, were photographed side 

 by side on the same plate with a Littrow 

 or auto-collimating spectrograph of 18 feet 

 focal length, used in conjunction with the 

 Snow telescope of the Mount Wilson Solar 

 Observatory. The third or fourth orders 

 of a 4-inch plane grating, having 14,438 

 lines to the inch, were employed. For the 

 measurement of line displacements, spectra 

 were photographed at points near the limb 

 lying at opposite ends of a solar diameter, 

 thus permitting the rotational shifts to be 

 eliminated. Some of the more recent work 

 has been done with a Littrow spectrograph 

 of 30 feet focal length, used with the new 

 vertical coelostat or "tower" telescope of 

 the Solar Observatory. This instrument 

 is of the same focal length as the Snow 

 telescope (60 feet), and thus the diameter 

 of the solar image is about 6.7 inches in 

 each ease. The 4-inch grating, when used 

 with the 30-foot spectrograph, gives a 

 scale of 1 mm. :^ 0.58 Angstroms in the 

 third order and 1 mm. ==0.44 Angstroms 

 in the fourth order. As the Fraunhofer 

 lines are fairly sharp on the photographs, 

 this great scale permits a high degree of 

 precision to be attained in their measure- 

 ment. Up to the present time, most of 

 the work has been done in the region 

 A 3,800 — A. 5,800. It is therefore quite 

 possible that the preliminary results given 

 in this paper may not apply below D or in 

 the ultra-violet. These results may be 

 summarized as follows: 



1. Most of the lines shown by our photo- 

 graphs of spot spectra to be strengthened 

 or weakened in sun-spots, are similarly 

 affected near the limb. 



2. Many lines not affected in spots are 

 strengthened or weakened near the limb. 



3. Lines due to substances of high atomic 

 weight are, in general, greatly weakened 

 near the limb. 



4. "Winged lines undergo marked change 

 in appearance, the wings being greatly 

 reduced near the limb. 



5. Among the lines considerably 

 strengthened near the limb the most im- 

 portant are due to elements of compara- 

 tively low atomic weight. These include 

 the D lines of sodium, the i lines of 

 magnesium and the blue calcium line at 

 A 4,227. 



6. Most of the lines in the spectrum are 

 slightly widened near the limb. 



7. Most of the lines are shifted toward 

 the red, as compared with their position 

 at the center of the sun. 



8. These displacements are not due to 

 ascending currents at the center of the 

 sun (which would produce negative dis- 

 placements of the lines in the solar com- 

 parison spectrum), since they have also 

 been measured with the aid of an are 

 comparison spectrum. 



9. The magnitude of the shift varies 

 for different lines of the same element. 



10. The strengthened lines, as a rule, 

 seem to show smaller shifts than do the 

 other lines. 



11. The spark lines of a given element, 

 as a rule, show larger shifts than do the 

 other lines. 



12. In many cases the relative displace- 

 ments of the lines agree fairly well with 

 those obtained by Humphreys in his 

 laboratory experiments on the effect of 

 pressure on wave-length. 



13. The lines of the cyanogen flutings 

 (A 3,883.5 and A 4,216.14) are not shifted 

 from their normal positions. 



14. The shifts of groups of titanium 

 lines near A 3,900, A 4,500 and A 5,300, and 

 of groups of iron lines near A 3,800, A 4,400, 

 A 4,900 and A 5,500, show progressive in- 

 crease toward the red, and seem to indicate 

 that the average pressure shift, for similar 

 lines, is a function of the wave-length. 



