He showed a particular  interest in  astronomy,  through his experiments related  to the determination  of the speed of the light, his research on the solar spectrum, the terrestrial magnetism, and the  average density of the Earth,  to which he assigned  a  remarkably accurate  value   of  5.53 . 

   The  year after, in 1874,  he  determined the speed of the light between the Observatory of Paris and the Tower of Montlhéry  using   the method, known as   the toothed wheel method,  that was formerly   described  by Fizeau,  but  that  he   improved  significantly.  The speed that he  found,   300.400 kms-1, corresponded to an  improvement  with respect to  previous  measurements. For this achievement,  he won  the Lacaze Price of physics, of the Academy.  He  renewed this measurement at the Observatory of Nice, in 1902, with Perrotin   improving  again its precision. 

  Taking up an idea of John Herschel, according to which one should  vary the distances between the lenses, to overcome the chromatism of an  objective,  he  obtained beautiful photographs of the Moon, which was, still at that time, extremely difficult. 

    In spectroscopy, his  accurate  determinations of   the wavelengths provided a remarkable checking of the Balmer  formula.  Having successively studied  the ultraviolet  and infra-red parts of the spectrum,  he  drew out  a method, based  on the Doppler-Fizeau principle, to  separate the telluric lines from the purely solar lines. Also   interested in photometry, Cornu suggested the observation of an half-eclipse of the Jupiter satellites,  rather than the  observation of  their  total   disappearance. 
    He  had a great dexterity and remarkable technical skills. Many of  his research and   experiments were realized with  instruments  made by  himself or that were  built under  his  directives. 
    One  owes  to him a   number of improvements  brought  to the micrometers, the spectroscopes, the photometers,  and  polariscopes.

  He was also interested    in clock engineering,  especially the synchronization of   clocks and he  studied the effect of the terrestrial magnetism on the rate/rhythm of a 
stop watch. 

In his  scientific notes  he  also popularized clearly and simply  the results of the best scientific work of his time.

He  left  his name to a  famous  curve  that allows the calculation of the light intensities in the diffraction of Fresnel:  the Spiral of  Cornu. 

Member of the Academy of Science (1878)
Member of the Office of Longitudes (1886)
Foreign member then associate member with Royal Astronomical Society. 

Raymonde BARTHALOT