cyanometer, c. 1789, an instrument that measures the blueness of a sky
cyanometer, c. 1789, an instrument that measures the blueness of a sky
“But how to measure ‘blueness’? Using suspensions of Prussian blue, Saussure dyed paper squares every shade of blue he could distinguish between white and black. These were assembled into a numbered colour circle that could be held up to the zenith at a standard distance from the eye - the matching square established the degree of blue.“
http://www.rsc.org/chemistryworld/Issues/2010/October/SaussuresCyanometer.asp