The Henry Rey medal was founded in 1922 by Henry M. Rey from Marseille.
The prize is donated to the outstanding performance achieved in astronomy. According to the Wikipedia page of the French Astronomical Society, the first was awarded in 1930 and the medal is made of silver. This is in contradiction to the medal on the picture as that was made of bronze.
On one side of the medal - comming from the commemorative medal of the French Astronomical Society - the well-known portrait of Flammarion can be seen, a work of Alpheé Dubois.
On the reverse a laurel wreath, the name of the award: "PRIX HENRY REY", below it the name of the awarded and the year of awarding of the prize is shown. In this case it was received by R. CAILLAT in 1989. HD initials at the bottom of the reverse refer to the medalist's name.