Cyanometer

A cyanometer by Horace-Bénédict de Saussure (from the collection of Musée d'histoire des sciences de la Ville de Genève)
An artwork in Ljubljana, Slovenia, inspired by a cyanometer

A cyanometer (from cyan and -meter) is an instrument for measuring "blueness", specifically the colour intensity of blue sky. It is attributed to Horace-Bénédict de Saussure and Alexander von Humboldt. It 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.