GETTYSBURG, Pa. — The wheat had been flattened in the somber field where the dead Confederates were lined up for burial in 1863.

Forty-four bodies, some with their legs tied together to make them easier to carry, had been gathered by their comrades. But there was no time to dig the graves, and this was how the photographers found them, laid out on the trampled ground.

William A. Frassanito, the reclusive historian of Civil War photography, is standing in the woods just outside the field at sunset, explaining how he located this spot after it had been lost for more than a century.

It’s quiet now, except for the cooing of mourning doves and the lowing of cattle that graze in the knee-high grass....