LIKE MISSING PIECES FROM A VAST JIGSAW PUZZLE, SCRAPS OF HISTORY keep appearing in unexpected ways. Sometimes they raise more questions than they answer. Scuffed and dim, their delicate unprotected surfaces badly scratched, these daguerreotypes of the Mexican War, taken in 1847, were acquired a few years ago by the Amon Carter Museum of Fort Worth, Texas. Thirty-eight of them altogether, carelessly wrapped in wadded paper towels, were found in a Connecticut barn belonging to a family with no known link to the war in Mexico. Oddly enough they are very like the only other known photographs of this first war ever recorded by a camera, nine similar scenes that came to Yale more than thirty years ago and are now in that university’s Beinecke Rare Book and Manuscript Library. Their provenance, too, is lost. Did the same man make them? Who was he? Where have they been all these years? That is the next challenge, and the answer, if any, will probably come by accident, leaving still other tantalizing questions.