How to measure blueness?

Horace-Bénédict de Saussure (1740-1799), Image of the original cyanometer

What a curious idea we might think today. Why should one measure the sky, why press its shades of blue into categories?

Aristotle was probably the first to study the color of the firmament. And Leonardo da Vinci recognized that blue cannot be an intrinsic color of the air.

Horace-Bénédict de Saussure (1740-1799) too looked at the blue of the sky with the eyes of a scientist. During his climbing tours, he noticed something strange. He found that on high mountains the sky appears a far darker blue than on the plains. Farmers in the Alps rumored that the sky would eventually turn black with increasing altitude, drawing the unwary wanderer into a void. Such eerie legends should probably keep people away from the peaks.

When Saussure reached the top of Mont Blanc in 1787, he compared the sky to 16 different colored cards and was not yet satisfied. A year later, he held his new invention up in the cool mountain air of the Col du Géant: a circular disk with an opening in the center called a ‘cyanometer’. Using suspensions of Prussian blue (the first artificial pigment), Saussure dyed paper squares every shade of blue he could distinguish between white and black. These were assembled into a numbered colour circle with 53 sections that could be held up to the zenith at a standard distance from the eye – the matching square established the degree of blue. Saussure wanted to use it to determine the color of the firmament. He wanted to determine scientifically: how blue is the sky?

Simply enjoying the blue sky would not have occurred to the restless naturalist. After all, he was a man of his time, a man of the periode of enlightenment. He measured all he could, and when he couldn’t measure it, he designed new instruments to do so. He concluded, correctly, that the color of the sky was dependent on the amount of particles, water droplets and ice crystals, suspended in the atmosphere.

By the way, the darkest blue was measured by Alexander von Humboldt: 46 degrees above the Ecuadorian volcano Chimborazo. A darker blue was never measured with the cyanometer. Despite such prominent proponents, the cyanometer never caught on. And Saussure rushed on, from one research project to another.

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