National Geographic : 1956 Dec
786 Mount Wilson and ralomar uoservaSores SRed Plate Displays the Milky Way's Starry Host + Blue Takes a Thin Census Sky Survey astronomers photographed each celestial area twice, on red-sensitive emulsion (above) and blue-sensitive (below). Interstellar dust blacked out many star images on the blue plate. An identical red exposure pierced the barrier and recorded more stars, revealing the amount of dust. Luminous clouds of hydrogen were invisible on blue plate but dazzling bright on red. Both photographs show a section of Sagittarius (the Archer, ninth constellation of the zodiac), a region in the Milky Way.