The Donohoe medal was established to reward those who discovered a comet.
The medal was issued by the Astronomical Society of the Pacific, a scientific and educational organization, founded in San Francisco on February 7, 1889.
Citation from paper Mercury September/October 1989, (see full text at the link below): "Joseph A. Donohoe, a wealthy San Francisco businessman, had donated $500 to endow a bronze "Comet Medal", to be awarded to the discoverer of each new comet. The directors accepted his gift with thanks, named him a life member, and laid out a set of guidelines for the award of the medal. It was given to 'the actual discoverer of any unexpected comet', who was expected to notify the director of Lick Observatory, giving the exact time of discovery, the comet's position and motion, and its appearance. ... The first medal was awarded in March of 1890 to W. R. Brooks of Geneva, New York. From them on several medals were generally given each year. Donohoe died in 1895, but the Comet Medal Fund supported 250 awards until 1950, by which time comet discoveries had become so frequent that the Donohoe Comet Medal had to be discontinued"
On the obverse of the medal a comet among the stars can be seem. Legend around is: "ASTRONOMICAL SOCIETY OF THE PACIFIC". Donohoe was in Paris during the design phase of this medal in the summer of 1889, so it is possible that letters A. D. on the obverse below the rosette refers to the famous French medalist Alphée Dubois.
On the reverse seven lines are engraved to the die: "THIS MEDAL, FOUNDED A.D. MDCCCXC BY JOSEPH A. DONOHOE IS PRESENTED TO" and "IN COMMEMORATION OF THE DISCOVERY OF A COMET ON". The empty line in the center holds the name of the discoverer, while at the bottom shows the date of discovery. These lines are engraved one by one into each struck medal, that's why they look negative.