MOUNT WILSON SOLAR OBSERVATORY. 269 



wave-length determinations for the hnes of the arc spectrum of iron. 

 Direct measurements made at Mount Wilson show that for 45 selected 

 pairs of lines the separation, within the limits of error of measurement, is 

 identical in the sun and arc spectra. 



The sun-arc displacements, as deduced from several hundred lines, 

 are the same for those with companions as for free-standing lines. 



The influence of an electric field (Stark effect) upon the spectrum 

 of lithium and calcium has been investigated in the laboratory. The 

 longitudinal components in the case of lithium are clearly polarized, 

 differing in this respect from those of hydrogen and helium. The 

 H and K lines of calcium show longitudinal components with a separa- 

 tion comparable to those for H, He, and Li, but are not polarized. 

 Negative results with the dispersion employed have been found for 

 Fe, Ni, Al, Mg, Zn, and Sr. 



Direct photographs of Jupiter and Saturn made with the 60-inch 

 reflector, using color filters transmitting infra-red, yellow, violet, and 

 ultra-violet light, show marked differences in the distribution of the light 

 of different colors on these planets. In the case of Jupiter, numerous 

 differences are found in the appearance of the dark and bright belts, 

 the former being more prominent in the Ught of longer wave-length. 



Photographs of Saturn show remarkable variations of the markings in 

 light of different colors. A broad dark belt surrounding the planet's 

 equator is visible in violet and ultra-violet light which is absent in 

 yellow light, and the same applies to a dark polar cap of considerable 

 size. Density measurements with a Hartmann microphotometer fully 

 confirm these results. 



The investigation of the probable relationship between Unear motion 

 and absolute magnitude for stars of several spectral types has been 

 continued during the year. 



The continuity in the series of the B and A, and the second-type 

 stars, has been established. The great majority of the B stars are 

 found to belong to the First Stream, which seems to be accelerated. 



The determination of the individual parallaxes of the brighter B-type 

 stars between the limits galactic longitudes 150° and 216° and 

 galactic latitudes —30° and -f30° has been completed. 



Several long-exposure photographs, mainly of spiral nebulae, have been 

 obtained during the year. 



Measurements upon five photographs of Messier 101, representing 

 intervals of 5, 9, and 15 years, give strong evidence of internal motion. 

 The results agree in indicating a motion, at a distance of 5' from the 

 nucleus, of O'f 022 annually in a tangential direction, and of 0':f007 in a 

 radial direction outward. These results may be explained by a rota- 

 tional motion, or by movement outward along the arms of the spiral. 

 If interpreted as a rotation, the period would be about 85,000 years. 



Complete parallax determinations have been made for 15 stars. 

 The mean probable error of a final parallax is =t=0T006. 



