A half-eaten Golden Delicious apple, abandoned at the scene of a fire, made legal history this month in the Court of Criminal Appeal when Karl Johnson, a garbage man failed to have his conviction for arson quashed. The court held that the telltale marks of his teeth on the apple were evidence as valid as a fingerprint.

It was the first time that bite marks have been the sole evidence of indentification in a British criminal case, and it was enough for Johnson to be sentenced to three years in prison. Remarkably, the bite in the apple provided 46 points of similarity with Johnson's teeth, far more than in most fingerprint indentifications.

There was no other shred of evidence pointing to Johnson's involvement in the crime. He had been caught by the science of forensic odontology - a little-known branch of dentistry that studies the often gruesome evidence of bite marks at the scene of a crime.

The fire, at the North-West Water Board's offices in Southport, a seaside resort city north of Liverpool, destroyed most of the building, causing $50,000 in damage.

But in one unburned office, police found an apple with a single bite in it. They rushed it through Grand National traffic jams to the Liverpool home of John Furness, the world's leading authority on bite marks, who is consultant dental surgeon to the Home Office and a lecturer in forensic odontology at Liverpool University. Furness quickly bottled the apple in preserving fluid.

Police picked up Johnson five days later. His home was only a few streets away from the water board offices, and his criminal record included and incident where fire had been used.

Johnson's alibi was that he had been with friends until after midnight, when he had returned home to watch, on television, the end of the film "Cat Ballou." However, he agreed to have impressions taken of his teeth.

The evidence of John Furness showed that Johnson was the man who had bitten the apple - which, it turned out, had come from a secretary's office drawer, where it had been left uneaten the afternoon before the fire.

Among the 46 points of similarity Furness found was that the unsual round shape of one tooth - in most cases, teeth are square-shaped - was replicated in the apple, where the bite marks matched Johnson's lower jaw was slightly forward, matching another section of the bite marks. "Teeth Cannot Lie"

THE COURT of criminal Appeal, considering Johnson's case, looked at the whole status of forensic odontology. Justice Mars Jones said that it was now an established science, on a par with fingerprint and fiber evidence. In the orginial trial, the judge had also stressed tht bite-mark evidence was as admissible asfingerprints.

Johnson's counsel argued in vain that odontology was new, untried and little more than a hobby. But the forensic odontologists claim that, with 32 teeth in a full set, the odds against two people having identical teech are, as with fingerprints, 2.5 billion to 1.

Case involving bite marks have been known since 1906, when teeth marks in a piece of cheese were part of the evidence against a Cumberland collier found guilty of burglary. In 1960, however, a London magistrate warned it would be "utterly unsafe to convict on the evidence of bite marks alone."

Since then, John Furness has been one of a small group of dentists pioneering the increasingly sophisticated study of bite marks. Over 17 years, he has supplied forensic evidence in more than 100 cases.

One classic case was the Linda Peacock strangling in Lanarkshire in 1968. From a bite found on the girl's breast, police identified the killer from 29 suspects.

In 1970, Manchester police retrieved a piece of meat pie overlooked at the scene of a murder. The teeth marks on it helped to convict the murderer.

Furness explains: "No two people's teeth wear in the same way. just as each blacksmith will use his hammer in a slightly different way, and a criminal will use his jimmy in a particular way, so no two people chew in the same way. The teech take on unique characteristics. People can lie through their teeth, but their teeth cannot lie."

This article is reprinted from The London Sunday Times.