The Prints by Gary Savage. Excerpted from JFK: First Day Evidence.
[Dallas Police Crime Lab Detective R. W. (Rusty) Livingston] has copies of five photographs taken by Lieutenant Day made directly from the original Dallas police negatives which show
latent fingerprints found on the trigger housing of the Mannlicher-Carcano rifle from the sixth floor of the Depository. The fingerprints are visible to the naked eye even before enhancement. Each of the fingerprint photographs was taken with a light shining on the trigger housing from different directions in order to produce various contrasts of the fingerprints. This was an attempt by Lieutenant Day to bring out as much of the ridge detail as possible in order to do a (comparison for identification of whoever had previously handled the rifle (the shooter). Fingerprint ridges are the lines running around each finger from one side of the nail to the other. The raised ridges are unique to every person.
The rifle was completely covered with black fingerprint powder by Lieutenant Day in order to check for prints after he had returned to the Crime Lab around dusk on the evening of the assassination.
The fingerprint photographs which Rusty retained copies of should not be confused with the
palm print that Lieutenant Day
found underneath the barrel of the disassembled rifle. This evidence is in addition to that. Many studying the assassination have confused the issue of what prints were found on the rifle as well as where and who actually found them.
http://www.jfk-online.com/prints.html