An old Shakespearean once told Jon Finch -- who was nervous about acting in verse -- that if he ever dried up on-stage, all he had to do was declaim this great thundering Elizabethan line:

"Swing back the burner to its full extent!"

The prompter, having been warned that this meant the actor was in trouble, would hasten to whisper the next speech.

"And you know, it worked beautifully," said Finch, who plays the title role in "Henry IV, Part I," tonight at 8, and "Henry IV, Part II," April 9 from 8 to 11 p.m., both on PBS. "I was in "The Merchant of Venice,' and I dried, and so I boomed out this line. The prompter instantly came to my rescue."

Later he asked where the all-purpose line came from. Beaumont and Fletcher perhaps?

"Oh," said the old Shakespearean, "it's written in red letters on the back of my gas cooker. I could never possiblity forget it."

Most British actors grow up on Shakespeare, train at the Royal Shakespeare Company, move on to the National, and only then abandon the iambic pentameter for spy movies and TV thillers. Finch made his name in TV series, horror movies and "Sunday Bloody Sunday," taking on his first Shakespearean role as Macbeth at the age of 30 in 1971.

Roman Polanski has said Finch was his first and only choice to play the tyrant. He does Macbeth as a tightlipped, able young man consumed by his own ruthlessness: the ultimate hard-driving young executive.

"Roman wanted me to do the screen test, and I thought probably I'd get Donalbain or somethin, but when he called me in Paris and said I was Macbeth, I almost fell over."

It was Polanski, Finch's favorite director, who encouraged him with the verse speaking, which is as hard as acting in a foreign language. Technicians can help with the thees and thous (even old hand Anthony Quayle fluffs some of those), but only an actor who handles Elizabethan verse with utter freedom can make it sound natural to a modern audience.

After "Macbeth" came a lead in Hitchock's "Frenzy" and in "Death on the Nile." Film projects kept him racing all over the world; a TV series in Australia, a major international picture in Spain, where he lives half the year ("in the south, in the spaghetti western country"), and a lot of near-misses.

"I was going to have the part in 'Alien' where the monster chews its way out of my chest, but I was taken ill the first day. I also missed doing 'Flash Gordon,' which I gather is a disaster."

Then there was the Alistair Maclean auto racing film that never happened, the scuttled western with Yul Brynner, the "Hamlet" project he had to turn down because of conflicts.

"Finally I said, Sod it. I had the money, so I went off to race for two seasons. I got my international license, and my brother and I raced a Formula One car. But then I got diabetes, and I had to stop."

Meanwhile, the Polanski "Macbeth" so scorned by its distributors that only six prints exist today, was making Jon Finch famous as a Shakespeanean actor. The Royal Shakespeare asked him to do Coriolanus. Offers came in steadily.

"I was terrified. I knew it would take time to build confidence in myself.

But I taught myself, as I taught myself everything except riding and fencing."

When he played the young Bolingbroke, the furture Henry IV, in "Richard II" as part of the BBC-TV Shakespeare cycle, even Sir John Gielgud, perhaps the greatest verse speaker of his day, commented on Finch's wonderful command of the difficult form.

From there, it was only natural that Finch be asked to pursue the powerful but gradually deteriorating character of Henry through the two parts of "henry IV." He was a sensation in England, where the cycle plays a year ahead of the United States.

Now he's touring the country to push the show. He says he enjoys the pace -- coast to coast in nine days, including 24 hours in Washington before flying on to Columbia, S.C., and finnaly home.

"I hate being out of work," he said. "Of course, in my business you can choose not to work. You can take off and read for a couple of months if you want. But I like to keep at it. Though I hope I never have to do another sodding TV series, I suppose I'll have to, one day."

Born in London, he was evacuated to the country as a small child during the Blitz, fled back to the city as soon as he was old enough. Entered the London School of Economics but quit after 10 days. Served in the paratroopers. Planned a world vagabonding tour for lack of something else to do but changed his mind and took a job as an assistant stange manager for about $9 a week.

The critial moment came when a lead in Tennessee Williams' "Night of the Iguana" took sick, and Finch, stage manager and understudy had to go on, instant American accent and all. He got the bug.

Now he's talking about still another project -- a film of Mervyn Peake's towering, half-mad gothic fantasy, the Gormenghast trilogy.

"What parts there are!" he flashed."The faces! The scenes!"

But it's Shakespeare, his new discovery, who excites him most.

"There are so many rotten scripts around, so many perfectly terrible stupid plots. And Shakespeare is always beautiful. Everything he does. Unfortunately, most producers don't realize that. They pick a star and then find a story, any story."

Mind you, he's not complaining about Shakespearean producers. They're not about to run, out of stories.

"I always get lumbered with kings," he said. "Now, Hamlet. . . I wonder. If I could get a shot at Hamlet. . . ."