In 1920, on the morning after Jascha Heifetz made his London debut, George Bernard Shaw (a great music critic along with all his other achievements) sent the teen-age violinist a letter.

"If you provoke a jealous God by playing with such superhuman perfection," Shaw warned Heifetz, "you will die young. I earnestly advise you to play something badly every night before going to bed, instead of saying your prayers. No mortal should presume to play so faultlessly."

Never were Shaw's keen but erratic prophetic powers more mistaken. After a long career characterized by faultless playing, Jascha Heifetz died yesterday, less than two months short of his 87th birthday.

Could he have played something badly every night before he went to sleep? Nobody who knows Heifetz could ask that question. He was incapable of playing anything badly on the violin, though he might have have gotten off an occasional imperfect note on the accordion, the Chinese lute, the samisen or other unusual instruments that he sometimes picked up and played just for fun.

There might be arguments over whether he was the greatest violinist of the 20th century, which his life came close to spanning. If greatness includes a show of emotional intensity, some would deny it to Heifetz; he was the first of the deadpan virtuosi who have become a feature of the 20th century, appearing totally unruffled even when he was playing music of ecstasy or heartbreak. In this, like Horowitz at the piano and Toscanini on the podium, he helped to establish a new trend of "objectivity" in musical interpretation that has dominated our concert life for decades.

Throughout Heifetz's career, the word "perfection" was often paired with a charge of "coolness" in evaluations of his work. It is relative, of course; he came into music history at a time when the most popular violin soloists loved to wear their hearts on their sleeves -- visually as well as sonically. He never played to catch the audience's eye, and he refused to exaggerate the feelings in the music. But most well-informed ears find plenty of emotion in his playing -- the emotion contained in the music itself, not slathered on like some kind of makeup.

But whatever criticisms were aimed at him, everyone -- emphatically including other violinists -- agreed that Heifetz was the most technically perfect violinist of the century.

It had been that way since 1910, when he was 9 years old and auditioned for the great Leopold Auer, who taught a whole generation of outstanding Russian violinists, including Mischa Elman, Efrem Zimbalist Sr. and, briefly, Nathan Milstein.

Auer, to whom Tchaikovsky had dedicated his violin concerto, was amazed at young Heifetz, who became the most famous of his many outstanding students. "I have been teaching for 45 years," he said, "but this is the first time I have heard a violinist of such remarkable quality." Once, hearing Heifetz play Paganini's "Motu Perpetuo," Auer winced. "He doesn't even realize that it cannot be played that fast," he muttered almost inaudibly.

Fritz Kreisler first heard the 12-year-old Lithuanian native in Leipzig in 1913. He turned to Efrem Zimbalist with a remark that has become legendary: "You and I might as well take our fiddles and break them across our knees."

Isaac Stern and Sir Yehudi Menuhin, perhaps Heifetz's best known surviving colleagues, were eloquent in their praise on being informed of his death yesterday.

"Few people would disagree that he was probably the greatest violinist in our age," said Menuhin. "I would like to say that he was my greatest inspiration. I used to have all his recordings and tried to emulate him."

Stern said that "since his debut in the United States, {Heifetz} was the most powerful force in violin playing in the world. He has been in the inner ear of every violinist since at least 1930."

Heifetz's American debut, in New York Oct. 27, 1917, inspired a classic anecdote. All the New York musical establishment was there. Sitting together in a Carnegie Hall box were pianist Leopold Godowsky and violinist Mischa Elman, the first of Auer's students to achieve international fame.

As the concert proceeded, the audience was clearly amazed by this new talent, and its applause kept growing louder and louder. Finally, Elman allegedly turned to Godowsky and there ensued one of the great two-line jokes in musical folklore:

Elman: "Isn't it awfully hot in here?"

Godowsky: "Not for pianists."

Two years later, when he was only 18, Heifetz became the highest-paid violinist in the world, getting $2,250 per concert compared with $2,000 for the great Kreisler (who did not break his instrument over his knee).

"Here is my biography," Heifetz once summarized. "I played the violin at 3 and gave my first concert at 7. I have been playing ever since."

His career went at high speed for half a century, began to taper off in the '50s and was phased out in the 1960s, when he began to emphasize chamber music with cellist Gregor Piatigorsky rather than the virtuoso solo repertoire.

"I've had my share of touring," he said in a 1968 interview. "I have no further interest in that kind of career. And I can't say I admire the pace at which today's musicians travel. They move too fast; they play too often; they don't pause to reflect."

A concert given in 1972 was recorded, and it shows him still close to the height of his technical powers, though at the end he told his audience, "I'm pooped." Unimpaired mastery of his instrument can also be seen in a 1971 television film made in Paris. Filled out with interviews and scenes of Heifetz relaxing at his home in California and teaching his students, the film played several times to great acclaim on American television. It is now available on videotape from Video Arts International.

His art survives also in numerous recordings, virtually all made for RCA Victor. His best work is being reissued now on compact discs, giving him a new surge of attention. One RCA compact disc, containing his recordings of both the Beethoven and the Brahms violin concertos, has already become almost legendary among music lovers and seems impossible for record stores to keep in stock.

On his 80th birthday, Feb. 2, 1981, he refused to take part in any celebration. But in Lincoln Center, Itzhak Perlman turned to the audience in the middle of a concert and delivered a brief, in absentia tribute, calling him "the greatest violinist who ever lived." Perlman's vote makes the verdict unanimous from three or four generations of great violinists, whose lives span at least a century and a half. The earliest ones, Auer and Kreisler, encountered him before he reached his teens. Many of the youngest have heard him only on records, which have certainly reinforced the reports of his technical perfection. And with film, videotape and compact discs, he is likely to remain an influence for the foreseeable future.