Harold D. Babcock

MAPPING THE MAGNETIC FIELDS OF THE SUN

New apparatus for observing magnetic fields on the surface of the sun has recently been put into use at the Hale Solar Laboratory in Pasadena. It utilizes the longitudinal Zeeman effect, wherein a spectral line is split by the field into two overlapping oppositely polarized components. The new equipment has three noteworthy features : (1) a large plane grating superior to those that have been available previously; (2) a photoelectric detector having two slits symmetrically placed on the wings of the chosen line, where the profile is steepest, with two photomultipliers connected to a difference amplifier; and (3) provision for scanning the suns disk and mapping with a cathode-ray tube and a camera the intensity and polarity of the magnetic fields on the surface. The new plane grating used in the 75-foot underground Littrow spectrograph has a ruled area 13 by 20 cm with 600 grooves per mm, and is blazed in the fifth order green, where the dispersion is 1 1 mm per angstrom and the resolving power, measured photographically, is 600,000. Scattered light and ghosts are entirely negligible. The resolving power of this grating is ample to render line profiles with high accuracy, so that the interferometer, with its critical adjustments and low light transmission, can be eliminated.

THE RELATIVE NUMBER OF LINES OF DIFFERENT INTENSITY IN THE SOLAR SPECTRUM

For statistical purposes, the Revision of Rowlands Preliminary Table of Solar Spectrum Wave-Lengths was divided into 45 sections containing approximately equal numbers of lines. In those sections the atomic solar lines of the various intensity classes were counted, those of intensity exceeding 3 being grouped together. All lines known to be of molecular origin, either solar or telluric, were purposely omitted in order to make the comparison with laboratory data more significant, although the large number of weak lines still unidentified in the Sun leaves some uncertainty in this regard. The total number of lines counted from I 2975 to A 10218 was 18,423. Beyond the limit of Rowlands work, A 7330, the intensity scale is necessarily somewhat different, chiefly with respect to the weakest lines. Since infra-red photography is increasingly difficult for the longer wave-lengths, lower dispersion and resolving power must be used for this region, with the resultant loss of many of the faintest lines. Published intensities of infra-red solar lines

Photography of the Infra-red Solar Spectrum

NEARLY fifty years ago, Sir William Abney (Phil. Trans., Part II., p. 653, 1880, and Part II., p. 457, 1886) photographed and measured fine detail in the solar spectrum out to λ9867. He also recorded, with low resolving power, a few broad absorption bands of greater wave-length, but he evidently observed no individual absorption lines having wavelengths exceeding that of the line mentioned. It is remarkable that, in spite of some subsequent improvements in equipment, no one has measured lines in the solar spectrum out to the limit reached by Abney. The nearest approach of which I am aware is that of Brackett (Astro-physical Journal, 53, 121; 1921), who measured λ9849 and could see a few more faint lines beyond.