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 5.6K, 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 3.18K, utilizing essentially the same procedure but with better data.

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30.

That is, T=0K=273.15°C.

31.
Gustav Robert Kirchhoff (1824–1887) was a German physicist, and his scientific contributions were mainly in the field of electrical circuits, spectroscopy, black-body radiation, and elasticity theory. He is the author of two fundamental laws of the classical theory of electrical circuits and thermal emission. Guillaume may be referring here to Kirchhoff's 1859 paper: “[…] we can very easily prove, from the general principles of the mechanical theory of heat, that for rays of the same wave length at the same temperature the ratio of the emissive power to the absorptive power is the same for all bodies.” In 1860, Kirchhoff introduced the term “black body” with the following definition:
The proof I am about to give of the law above stated rests on the supposition that bodies can be imagined, which
,
for infinitely small thicknesses
,
completely absorb all incident rays and neither reflect nor transmit any. I shall call such bodies
perfectly black
or
,
more briefly
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bodies
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39.

Wilhelm Carl Werner Otto Fritz Franz Wien (1864–1928), usually known as Wilhelm Wien, was a German physicist. In 1893, he used theories about heat and electromagnetism to deduce the so-called Wien's displacement law. This law states that the black-body radiation curve for different temperatures will peak at different wavelengths that are inversely proportional to the temperature. In 1911, he received the Nobel Prize in physics for his work on heat radiation.

40.

Benjamin Apthorp Gould (1824–1896) was an American astronomer who created the Astronomical Journal in 1849 and founded the Argentine National Observatory at Córdoba in 1868 and the Argentine National Weather Service. He was one of the pioneers in utilizing photography for the study of astronomy. Hamilton expressed Gould's formula with the following words: “each full magnitude would be the fifth root of 100 (about 2.512 times) brighter or dimmer than the next full magnitude.” Note that this is just the mathematical formula for astronomical magnitudes proposed by Norman Pogson in 1856.

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William de Wiveleslie Abney (1843–1920) was an English astronomer, chemist and photographer. In 1882 he received the Rumford Medal. In 1896 he published a paper on the photographic values of moonlight and starlight compared with the light of a standard candle.

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William Hyde Wollaston (1766–1828) was an English chemist and physicist. In 1820, he was president of the Royal Society.

47.

Pierre Bouguer (1698–1758) was a French geophysicist and astronomer. In 1729, he published a work defining the quantity of light lost by passing through a given depth of the atmosphere. He compared the intensity of the light of the Sun with that of the Moon and made some of the earliest measurements in photometry.

48.

Johan Karl Friedrich Zöllner (1834–1882) was a German astrophysicist at Leipzig University. He created several photometers for the study of the Sun, Moon, and the stars, which he called astrophotometers. In 1867, he made the first measurement of the Sun's apparent magnitude using a photometer of his own design, which superimposed the image from a small telescope on the image from a reference lamp.

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