[Skyblue - the cyanometer of Horace-Bénédict de Saussure (1740 - 1799)]

The cyanometer is a simply constructed measuring instrument that enables a determination of skyblue. It consists of a color-scale that is arranged circularly going in equal steps from white to blue (Prussian blue) and finally into black. According to its inventor--Horace-Bénédict de Saussure--the azure is determined by the amount of so called opaque vapors in the atmosphere associated with meteorological phenomena. As outlined by De Saussure, the blackness of the universe seen through an illuminated and blurred medium results in azure. Thereby his instrument offers a relative scale that is consistent with color theories of his time like those of Johann Wolfgang von Goethe. The description allows the construction of the scale without the employment of standardized color-prints. Instead he provides a clear report of the necessary procedures to produce such a scale. The accuracy of this description is tested and discussed employing the methods of experimental history of science. The reception of the cyanometer in the time about 1800 and its implications on color theories is discussed.