Complete and commented translation of Guillaume's 1896 paper on the temperature of space

Charles Édouard Guillaume (1861–1928) was a Swiss physicist who received the 1920 Nobel Prize in physics for his precision measurements and discovery of anomalies in nickel steel alloys. In this work, we present a complete and commented translation of his remarkable article of 1896 on the temperature of interstellar space. The importance of this work is that it is the oldest estimate known to us of the temperature acquired by a black body, which is in interstellar space far from other stars. This temperature was presumed to be due to an equilibrium state in which the radiation received by this body from the stars around it would be equal to the radiation emitted by the body. He arrived at a temperature of 5.6 K, regarding this figure as an upper limit on the effect he was seeking to estimate. In 1926, Arthur Eddington (1882–1944) arrived at a temperature of 3.18 K, utilizing essentially the same procedure but with better data.