The Cyanometer: measuring color the Antique way
The Cyanometer is a magical yet simple instrument for measuring ‘blueness‘, not the feeling but specifically the colour intensity of blue sky. A Cyanometer consists of squares of paper dyed in graduated shades of blue and arranged in a color circle or square that can be held up and compared to the color of the sky. It helped lead to a successful conclusion that the blueness of the sky is a measure of transparency caused by the amount of water vapor in the atmosphere. 
(Horace-Bénédict de Saussure’s cyanometer, 1760)
For a modern day meter
(by Martin Bricelj Baraga. Photocredits; Martin Bricelj Baraga)
Monolith in Ljubljana
The work in the image above can be seen “at Plateau Ajdovščina in Ljubljana. The Cyanometer by Artist Martin Bricelj Baraga is a monolith that measures the blueness of the sky and visualises the quality of air in Ljubljana, and simultaneously sends data to an online archive” via plugin-magazine.com
The invention of the Cyanometer is attributed to Horace-Bénédict de Saussure and Alexander von Humboldt, in 1789. De Saussure’s meter had 53 sections, ranging from white to varying shades of blue (dyed with Prussian blue) and then to black, arranged in a circle; he used the device to measure the color of the sky at Geneva, Chamonix, and Mont Blanc. De Saussure concluded, correctly, that the color of the sky was dependent on the amount of suspended particles in the atmosphere. (knowledge from Wikipedia)