found this description:
A FEDERAL COURT-MARTIAL AFTER GETTYSBURG The court-martial pictured is that of the second division, Twelfth Army Corps. It was convened at Ellis Ford, \a., in July,1863. Such officers were especially detailed from various regiments of a division of their corps, for the purpose of judging all classes of cases, crimes, and misdemeanors against the general regulations of the army. The officers above tried a large number of cases of desertion, insubordination, and disobedience to orders, sentencing in this particular court-martial three deserters to be shot. Two of these men were executed in the presence of the whole division, at Mortons Ford on the Rapidan, in September following. The idea of a court-martial in the service was somewhat similar to that of a civil jury. The judge-advocate of a general court-martial stood in the relationship of a prosecuting district-attorney, except for the fact that he had to protect the prisoners interest when the latter was unable to employ counsel.