These perfect autodidacts were born in a family too poor to allow them to make studies. After an elementary instruction,  they had to be formed by themselves. At the age of sixteen, they were assigned to the service of weather forecasting that Le Verrier had recently created at Paris Observatory.

    Le Verrier, struck by their genius for astronomical work, secured  their services. H offered a shed to them, where they could establish a workshop to train their ability. But it was in the small optical workshop  equipped by their own means in their own house, in Montrouge (south of Paris) that they preferred to make their researches.

    They undertook, the construction of a thirty centimeter mirror and his mounting. With this reflector and a second hand clock, during their leisures, they began  to draw a map of the stars in the ecliptic zone (1868).
 
   In 1871, Charles Delaunay, director of  Paris Observatory, heard of their work, and transferred them to the equatorial telescope section. The discovery of a small planet, named Liberatrix, owing to the political circumstances (the war between France and Prussia) inaugurated the  execution of a map  of the stars in the ecliptic zone aimed at helping  the quest  of small planets. The project was  to survey the positions of all stars down  to  the 13th   visual magnitude within a 5° wide sky band.

      By 1884, their survey  was carried out to the fourth part,  when they approached the crowded center  of the Milky Way  the confusion due to the   increasing  number of stars made visual observations almost impossible. Then, they thought of using the photography recently  improved  thanks to  the invention of the gelatino-bromide plates.

   The Henry brothers  gave up the ordinary refractors and devised a new instrument, better adapted to the aim.

  They  cut a sixteen-centimeter lens objective, achromatized for the wavelengths to which photographic plates are sensitive, and they coupled the photographic telescope  to a visual guiding telescope, thus becoming able to control precisely the tracking  of the equatorial telescope during the course of exposures lasting as long as one hour.
 
   The results were so good that  the Admiral Mouchez, director of Paris Observatory, charged them with the construction of an equatorial or astrograph able to carry out the photographic map of the Sky, which was still at the state of a  project.  This instrument, whose mounting was designed  by Paul Gautier, had a 34-cm, f/10 photographic reflector   and the guiding telescope, with an opening of 25-cm, was of the same length  (1885). This astrograph quickly revealed its qualities and they obtained amazingly successful results. With a 3-hour exposure, the Henry brothers recorded up to 1421 stars in  the Pleiades cluster region, whereas ten years earlier the American Lewis Rutherfurd, with a wet-plate process, had counted visually no more than 50. At the same time they discovered  the Maia Nebula, in the Pleiades, which was confirmed later visually with the Poulkovo 76-cm refractor.
  The success was due to a rare perfection of the optical work, realised by the Henry  brothers, and which was recognized with a real enthusiasm by David Gill, Director of  Cape Observatory. Gill urged on  Mouchez to organize an International Astrographic  Congress for the survey of the Sky chart (Congrés astrographique International pour le levé de la Carte du Ciel) which took place in Paris, in 1887. The Henrys' astrograph  was  adopted as the standard instrument, and more than half of the seventeen instruments  were  actually built  by the Henrys.
  Their name remain attached to this enterprise, but their ability went on to practise until their death, and they never ceased to make with their hands the optical parts of all the large French coudés and telescopes, such as those of Lyon, Paris,  Nice and  Meudon.  Their distinctive features were modesty, discretion, and abnegation.  

    The unexpected death of Prosper of a cerebral congestion during a journey  in the French Alps stopped the work of the elder. Paul was extremely affected and died  at his home  after his brother,  also carried off by   a cerebral congestion.
   
     They were awarded by the Prix Lacaze of the Académie des Sciences(1887), for all their works, and elected Associates of the Royal Astronomical Society(1899).