The question asked by Horace-Benedict de Saussure was not why the sky was blue, but how blue is it? This intrepid young scientist of the late 18th century puzzled as he observed the brilliant blue skies over the Alps. But true scientists do not merely puzzle and observe, they measure.
So Saussure devised this color-ring with every shade of blue between white and black that could be held at a standard distance from the eye to document the sky’s color at zenith. He called a “cyanometer.” Apparently he also measured humidity at high altitudes to come to the conclusion that the sky’s degree of blueness was a result of the amount of water vapor in the atmosphere. – It is reported that the darkest shade of blue he found was #39. Via Facebook.