Editing François Arago (section)

==Scientific studies==
Arago had succeeded in preserving the records of his survey; and his first act on his return home was to deposit them in the [[Bureau des Longitudes]] at Paris. As a reward for his adventurous conduct in the cause of science, he was elected a member of the [[French Academy of Sciences]], at the remarkably early age of twenty-three, and before the close of 1809 he was chosen by the council of the {{lang|fr|[[École Polytechnique]]|italic=no}} to succeed [[Gaspard Monge]] in the chair of [[analytical geometry]]. At the same time he was named by the emperor one of the astronomers of the [[Paris Observatory]], which was accordingly his residence till his death. It was in this capacity that he delivered his remarkably successful series of popular lectures in astronomy, which were continued from 1812 to 1845.<ref name="eb" />

In 1818 or 1819 he proceeded along with Biot to execute [[Geodesy|geodetic]] operations on the coasts of France, England and Scotland. They measured the length of the [[seconds pendulum]] at [[Leith]], Scotland, and in the [[Shetland Islands]], the results of the observations being published in 1821, along with those made in Spain. Arago was elected a member of the Bureau des Longitudes immediately afterwards, and contributed to each of its Annuals, for about twenty-two years, important scientific notices on astronomy and [[meteorology]] and occasionally on [[civil engineering]], as well as interesting memoirs of members of the Academy.<ref name="eb" />

Arago's earliest physical researches were on the [[pressure]] of [[steam]] at different temperatures, and the [[velocity]] of sound, 1818 to 1822. His [[magnet]]ic observations mostly took place from 1823 to 1826. He discovered rotatory magnetism, what has been called [[Arago's rotations]], and the fact that most bodies could be magnetized; these discoveries were completed and explained by [[Michael Faraday]].
[[File:Babbage, Charles – Account of the repetition of M. Arago's experiments on the magnetism manifested by various substances during the act of rotation, 1825 – BEIC 722599.jpg|thumb|left|upright|[[Charles Babbage]], ''Account of the repetition of M. Arago's experiments on the magnetism manifested by various substances during the act of rotation'', 1825]]

Arago warmly supported [[Augustin-Jean Fresnel]]'s [[optics|optical]] theories, helping to confirm [[Fresnel]]'s [[wave theory of light]] by observing what is now known as [[Arago spot|the spot of Arago]]. The two philosophers conducted together those experiments on the [[Polarization (waves)|polarization]] of light which led to the inference that the [[oscillation|vibration]]s of the [[luminiferous ether]] were transverse to the direction of [[motion (physics)|motion]], and that polarization consisted of a resolution of [[rectilinear propagation]] into components at right angles to each other. The subsequent invention of the [[polariscope]] and discovery of [[Rotary polarization]] are due to Arago. He invented the first polarization filter in 1812.<ref>{{The Timetables of Science|pages=261}}</ref> He was the first to perform a [[polarimetric]] observation of a comet when he discovered polarized light from the tail of the [[Great Comet of 1819]].<ref>{{cite book|last1=Kolokolova|first1=Ludmilla|title=Polarimetry of Stars and Planetary Systems|year=2015|publisher=[[Cambridge University Press]]|isbn=978-1-107-04390-9|page=380|url=https://books.google.com/books?id=KFqtCAAAQBAJ&pg=PA380}}</ref>

The general idea of the experimental determination of the [[velocity of light]] in the manner subsequently effected by [[Hippolyte Fizeau]] and [[Léon Foucault]] was suggested by Arago in 1838, but his failing eyesight prevented his arranging the details or making the experiments.

[[File:François Arago par Ary Scheffer.jpg|thumb|right|200px|François Arago]]

Arago's fame as an experimenter and discoverer rests mainly on his contributions to magnetism in the co-discovery with [[Léon Foucault]] of [[eddy current]]s, and still more to [[optics]]. He showed that a magnetic needle, made to oscillate over nonferrous surfaces, such as water, glass, copper, etc., falls more rapidly in the extent of its oscillations according as it is more or less approached to the surface. This discovery, which earned him the [[Copley Medal]] of the [[Royal Society]] in 1825, was followed by another, that a rotating plate of copper tends to communicate its motion to a magnetic needle suspended over it, which he called "magnetism of rotation"<ref>''Annales de chimie et de physique'' (1824), vol. 27, [https://books.google.com/books?id=r9U3AAAAMAAJ&pg=PA363 page 363]:  "M. Arago communique verbalement les résultats de quelques expériences qu'il a faites sur l'influence que les métaux et beaucoup d'autres substances exercent sur l'aiguille aimantée, et qui a pour effet de diminuer rapidement l'amplitude des oscillations sans altérer sensiblement leur durée. Il promet, à ce sujet, un Mémoire détaillé." (Mr. Arago orally communicates the results of some experiments that he has conducted on the influence that metals and many other substances exert on a magnetic needle, which has the effect of rapidly reducing the amplitude of the oscillations without altering significantly their duration. He promises, on this subject, a detailed memoir.)</ref><ref>{{cite journal | last1 = Arago | year = 1826 | title = "Note concernant les Phénomènes magnétiques auxquels le mouvement donne naissance" (Note concerning magnetic phenomena that motion creates) | url = https://books.google.com/books?id=Itg3AAAAMAAJ&pg=PA213 | journal = Annales de chimie et de physique | volume = 32 | pages = 213–223 }}</ref><ref name=Babbage1825>{{cite journal| last1=Babbage |first1=C. |last2=Herschel |first2=J.W.F. |year=1825 |title=Account of the repetition of M. Arago's experiments on the magnetism manifested by various substances during the act of rotation |journal=Philosophical Transactions of the Royal Society of London |volume=115 |pages=467–496 |url=https://archive.org/details/philtrans03806447 |bibcode=1825RSPT..115..467B |doi=10.1098/rstl.1825.0023|doi-access=free }}</ref> but (after Faraday's explanation of 1832<ref>[https://www.biodiversitylibrary.org/item/20031#page/300/mode/1up Philosospical magazine 1840]</ref>{{rp|283}}) is now known as [[eddy current]].  Arago is also fairly entitled to be regarded as having proved the long-suspected connexion between the [[aurora borealis]] and the variations of the magnetic elements.<ref name="eb" /> In 1827 he was elected an associated member of the Royal Institute of the Netherlands, when that institute became the [[Royal Netherlands Academy of Arts and Sciences]] in 1851, he became foreign member.<ref>{{cite web|url=https://www.dwc.knaw.nl/biografie/pmknaw/?pagetype=authorDetail&aId=PE00004595 |title=Dominique François Jean Arago (1786–1853) |publisher=Royal Netherlands Academy of Arts and Sciences |access-date=7 May 2020}}</ref> In 1828, he was elected a foreign member of the [[Royal Swedish Academy of Sciences]].

In optics, Arago not only made important optical discoveries on his own, but is credited with stimulating the genius of [[Jean-Augustin Fresnel]], with whose history, as well as that of [[Étienne-Louis Malus]] and [[Thomas Young (scientist)|Thomas Young]], this part of his life is closely interwoven.

Shortly after the beginning of the 19th century the labours of at least three philosophers were shaping the doctrine of the [[undulatory theory of light|undulatory, or wave, theory of light]]. Fresnel's arguments in favour of that theory found little favour with Laplace, Poisson and Biot, the champions of the emission theory; but they were ardently espoused by Humboldt and by Arago, who had been appointed by the Academy to report on the paper.<ref name="eb" /> This was the foundation of an intimate friendship between Arago and Fresnel, and of a determination to carry on together further fundamental laws of the polarization of light known by their means. As a result of this work, Arago constructed a polariscope, which he used for some interesting observations on the polarization of the light of the sky. He also discovered the power of rotatory polarization exhibited by [[quartz]].<ref>Arago (1811) [https://babel.hathitrust.org/cgi/pt?id=ucm.5326746608;view=1up;seq=103 "Mémoire sur une modification remarquable qu'éprouvent les rayons lumineux dans leur passage à travers certains corps diaphanes et sur quelques autres nouveaux phénomènes d'optique"] (Memoir on a remarkable modification that light rays experience during their passage through certain translucent substances and on some other new optical phenomena), ''Mémoires de la classe des sciences mathématiques et physiques de l'Institut Impérial de France'', 1st part : 93–134.</ref>

Among Arago's many contributions to the support of the undulatory hypothesis, comes the ''experimentum crucis'' which he proposed to carry out for measuring directly the velocity of light in air and in water and glass. On the emission theory the velocity should be accelerated by an increase of density in the medium; on the wave theory, it should be retarded. In 1838 he communicated to the Academy the details of his apparatus, which utilized the relaying mirrors employed by [[Charles Wheatstone]] in 1835 for measuring the velocity of the electric discharge; but owing to the great care required in the carrying out of the project, and to the interruption to his labours caused by the revolution of 1848, it was the spring of 1850 before he was ready to put his idea to the test; and then his eyesight suddenly gave way. Before his death, however, the retardation of light in denser media was demonstrated by the experiments of H. L. Fizeau and B. L. Foucault, which, with improvements in detail, were based on the plan proposed by him.<ref name="eb" />