Plate Number Secant of Plate Number 1 Secant of Plato Number Secant of and Exposure Zenith Distance and Exposure Zenith Distance and Exposure Zenith Distance 99b 2,48 131 c 1,64 165 c 2,22 100 b 3,62 132 a 1,74 166 a 1,78 101 c 1,55 135 a 1,59 „ c 2,02 102 c 1,55 „ b 1,60 167 c 2,02 103 a 1,53 136 a 1,59 231 h r 1,37 104 a 1,53 147 h 1,74 232 h 1,20 106 a 2,56 „ c 1,76 240 b 1,99 114 a 1,59 148 a 1,95 241 h 1,99 „ b 1,57 „ b 2,04 242 a 1,86 117 a 1,92 „ c 2,17 243 a 1,86 119 a 2,90 149 a 1,76 247 a 1,22 127 h 1,63 „ b 1,73 248 a 1,22 128b , 1,63 „ c 1,81 249 a 1,22 „ c 1,62 158 h 2,14 250 a 1,26 ‘ 129 a 1,94 159 h 2,14 251 a 1,26 130b 2,03 164 a 1,86 252 a 1,26 – – 131 a 1,74 165 a . 1,86 ' „ b 1,69 „ b 1,89 The Spectrograph. The spectrograph') of the 150-loot tower telescope is of the auto-collimating type with a focal distance of 22.9 m. It is equipped with a Michelson plane grating, No. 66, having about 600 lines to the millimeter and an available ruled surf ace of 6.7 by 12.5 cm. A field lens, which formed on the grating an image of the objective lens of the telescope, was placed about 80 cm above the slit of the spectrograph. The spectrum was photographed in the second order from A 3612 to X 6979 and in the first order from A 6924 to X 8771, the mean dispersion being 3.0 mm/A and 1.5 mm/A, respectively. Some other parts of the spectrum were photographed in the first order for comparison, but have not been enclosed in the Atlas. The width of the slit, chosen according to van Cittert's formula 2), was between 0.05 and 0.07 mm for the exposures in the second order and between 0.03 and 0.035 mm for those in the first order, depending on the wave length. The focus was determined with great care, especially in the ultraviolet, where it changes so rapidly with wave length that the plate holder has to be inclined. The exposure times in various regions of the spectrum ranged from 2 to 120 minutes. Photometric Calibration. Every plate has, in addition to the spectra for the study of line profiles, a set of six standard spectra for photometric calibration (Fig. 1). These were obtained by means of a step weakener. On account of slight astigmatism, unavoidable in a spectrograph of this type, the Fraunhof er lines and the edges of the standard spectra are focused in the same plane only if the step weakener is placed about 25 cm below the slit.
INTRODUCTION Absorption lines in stellar spectra are the main source of knowledge of the conditions in stellar atmospheres. The study of the distribution of light-intensity within the lines, in particular, yields wealth of astrophysical information as evidenced by a great number of investigations during the last fifteen years. Methods have been developed for a reliable determination of line profiles, the sources of systematic error have been discussed, and the limits of accuracy of modern observational technique indicated. The time seems opportune for a complete photometry of the line spectrum of a star. The sun is an obvious first choice for such an investigation because its spectrum con be obtained in great detail much more easily than that of any other star. The present volume records the solar spectrum from A, 3332 to  8771 in the form of a photometric Atlas, making the observed intensity profiles of all the Fraunhofer lines in that region directly available to every astrophysicist. Needless to say, this Atlas bas a preliminary nature: a survey cannot be performed with the same accuracy as the determination of a single line profile. A discussion of the errors that may be present in the results will be found on page 10. THE SPECTROGRAMS The solar spectrum was photographed at the 150-foot tower telescope 1) of the Mount Wilson Observatory, chiefly between September 1936 and April 1937. It was not possible to obtain integrated sunlight because the angular aperture of the spectrograph is smaller than the apparent diameter of the sun. Therefore it was decided to use the light from the center of the solar disk. The objective lens of the 150-foot tower telescope has a diameter of 30.5 cm and a focal distance of 45.7 m; it forms a solar image about 43 cm in diameter. To avoid the effects of local differences due to grctnulation, faculae, or small spots, the image was not focused on the slit of the spectrograph but in a plane about 1 m higher. In addition, the solar image was allowed to drift a few millimeters during the exposure. For the study of atmospheric lines the secant of the mean zenith distance of the sun for every plate between 5275 and A. 8771 is given in Table I. TABLE 1. SECANT OF ZENITH DISTANCE OF THE SUN FOR THE SPECTROGRAMS BETWEEN 2. 8275 AND 2, 8771 Plate Number Secant of Plate Number Secant of Plate Number Secant of and Exposure Zenith Distance and Exposure Zenith Distance and Exposure Zenith Distance 31 a 1,44 39 c 1,99 93 b 4,92 32 a 1,44 40 c 2,05 94 a 4,06 „ b 1,42 41 a 1,44 „ b 4,56 35 c 1,27 42 a 1,44 95 a 1,67 36 a 1,54 52 b 1,28 „ b 1,64 37 a 1,54 55 a 1,46 96 a 1,67 38 c 1,99 „ b 1,50 98 b 2,48
A PHOTOMETRIC ATLAS OF THE SOLAR SPECTRUM