Anyone who has gazed at a cloud free sky during daytime certainly havenoticed that the colour of the sky is blue. A careful observer may also havenoticed that the sky can look darker or lighter blue. The actual reasonsbehind this nature phenomena were not resolved until the second half ofthe 19th century but the variation of blue was already investigated duringthe time of Enlightenment by the Genovan scientist, aristocrat and mountainexplorer Horace Benedict de Saussure (1740{1799). De Saussure wasa scientist of his time and he measured many natural constants. When deSaussure climbed the Alp mountains he noticed that the sky turned darkerwith a more intense blue, at higher altitudes. To get a relative measure onhow blue the sky was, de Saussure invented a device he called cyanometer.It is a colour chart containing dierent hues of blue, starting with very lightblue, ranging all the way to dark blue. With this simple device de Saussurecould get a recordable measurement on how blue the sky was at the timefor the observation.This thesis deals with the cyanometer as a scientic instrument. It is ofinterest to investigate the cyanometer from a history of ideas perspective,so that this scientic instrument can be put in a visual culture context.Moreover, an investigation of how de Saussure developed the instrumentcan also show his thinking for the modications of it that he made. Further,the visual culture ideas can be applied on the main article de Saussure wroteabout the cyanometer in 1789.During the period when de Saussure was an active mountain explorer,he made several cyanometers which all varied in layout and the number ofblue colour sections included. In this thesis an analyse of the four dierentoriginal cyanometers that de Saussure made will be done. This analysis includesa quantitative comparison between the versions and a discussion onhow the instrument was made and operated. In order to trace a biographicalviewpoint on the cyanometer the thesis will also discuss how de Saussure'sidea, to measure the blueness of the sky, developed after his lifetime.