Source: Appendix X, Firearms And Firearms Identification General Principles
"In addition to rifling characteristics, every weapon bears distinctive microscopic characteristics on its components, including its barrel, firing pin, and breech face. While a weapon's rifling characteristics are common to all other weapons of its make and model (and sometimes even to weapons of a different make or model), a weapon's microscopic characteristics are distinctive, and differ from those of every other weapon, regardless of make and model. Such markings are initially caused during manufacture since the action of manufacturing tools differs microscopically from weapon to weapon, and since the tools change microscopically while being operated. As a weapon is used, further distinctive microscopic markings are introduced by the effects of wear, fouling, and cleaning."
When matching a suspect bullet to a weapon they compare the suspect bullet with several test bullets fired by the weapon. They use more than one test bullet because every bullet fired by a weapon has variations in its microscopic markings. The several test bullets serve as a measure of the variability of a signature. If the variations of signatures between the suspect and some test bullets are less than the larger signature variations among the test bullets, then they declare a match between the suspect bullet and the weapon.
Photographic identification is similar to firearm identification. Here they compare the suspect negative with test negatives exposed in the suspect camera. This is what the photographic panel of the HSCA intended to do.
Source: Addendum B Report to the House Select Committee on Assassinations U.S. Congress--House of Representatives The Oswald Backyard Photographs (By Dr. Leslie Stroebel, Mr. Andrew Davidhazy, Dr. Ronald Francis)
"(450) We had intended to make a quantitative comparison of the edge markings on the various photographs, as suggested by a panel member, by aligning parts of edge markings, measuring the displacement at fixed intervals and calculating the standard deviation. Careful examination of a roll of film we exposed in the Oswald camera revealed that while the distinctive marks appeared consistently on each frame of film, the straightness of the lines varied considerably-apparently due to slight buckling of the film. Instead, we made prints that compare pairs of edges on all four sides of the picture frame."
Although frames have common signature characteristics, each frame has variable microscopic features. These variable features have several causes. First the paper carrier of the film must be smaller than its guiding aperture. This tolerance permits unimpeded motion of the film and allows slight vibration during film advance. Next as they advance the film from frame to frame, the tension that pulls film from the supply chamber changes. Finally manual operation of the advance mechanism permits changes in speed and smoothness of motion. During scratching, the film emulsion abrades irregularities in the camera. This causes accumulating changes in the camera signature. Additionally mishandling a camera introduces new signature characteristics.
Linking a negative to a camera does not depend on matching the variable microscopic features. Instead they compare the common signature characteristics on a test negative with the suspect negative. If the common characteristics match then they prove the suspect negative bears the signature of the camera.
The variable microscopic features of the camera signature become important when two or more photographs have the same microscopic features. This leads to the paradox of the variable that reminds the same. We resolve this apparent contradiction by recognizing they did not make the three backyard photos from three different negatives bearing the varying signatures of Oswald's camera. Instead forgers impressed the same Oswald-camera signature on three different negatives that produced the backyard photos.
Return to New Leads in JFK Assassination Research