From The Science of Fingerprints, a U.S. Department of Justice publication
In order to take good fingerprints, the fingers must be clean and dry. The finger should not be inked too heavily nor pressed onto the paper too hard. To get a complete print, the finger should be rolled, starting from the awkward position and rolling to the easy position -- thus, the thumb is rolled toward and the fingers away from the center of the body.
The degree of pressure to be exerted in inking and taking rolled impressions is important. Relax! You should be able to see white between the ridges.
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