1. A device for determining the displacement of a tooth between two different examinations made on separate occasions, comprising a first photographic image of the tooth containing location points near the tooth taken on the occasion of the first examination, said first photographic image being in the form of a hologram, said first photographic image being placed alongside a second representation of the tooth having location points near the tooth taken on the occasion of a second examination, said location points on the first photographic image and the second representation being teeth adjacent said tooth, means for displacing the first photographic image relative to said second representation to move the location points on the first photographic image into correspondence with the location points on said second representation such that the displacement of the tooth is determined as the difference between the position of the tooth in the first photographic image and the position of the tooth in said second representation.