Six-color photometry of stars. I. The law of space reddening from the colors of O and B stars

The new photoelectric photometer described makes possible measures of stars and nebulae in six spec- tral regions from X 3530 to A 10,300 A. With the 60-inch reflector useful results can be obtained for stars as faint as the ninth magnitude. Measures (Table 3) have been obtained of the colors of 69 stars of spectrum 0 and B of various de- grees of reddening. The scale for the difference, ultraviolet minus infrared, is about five times the inter- national scale for color index. The deviations from the 1/X law of selective absorption (Table 6) are in the sense that the intermedi- ate regions are fainter than they would be if the 1/X law held from X 3530 to X 10,300 A. There is no reason to change the previous determination of a high ratio of total to selective absorption, Apg/Ejnt 6, but the numerical value of this ratio must remain uncertain in the absence of observations in the far infrared. The law of selective absorption is found to be the same for all directions in the Galaxy, indicating that interstellar material is essentially uniform in quality everywhere