18th century instrument to determine the sky’s ‘blueness’ called a Cyanometer: 

The simple device was invented in 1789 by Swiss physicist Horace-Bénédict de Saussure and German naturalist Alexander von Humboldt who used the circular array of 53 shaded sections in experiments above the skies over Geneva, Chamonix and Mont Blanc. 

via This is Colossal