WHEN THE announcement arrived in the mail last week that Barry Tuckwell was forming a new orchestra in western Maryland, the first impression was that it must be some kind of misunderstanding. Tuckwell setting up in Hagerstown as music director and conductor of a yet-to-be-created orchestra sounded about like Rampal settling down in Salisbury.

But at 2 p.m. Wednesday, there he was--the incomparable Barry Tuckwell, who has performed in hundreds more places than any other horn player. The next morning he would fly off to Mexico City and thence to Australia for several months--but for a few hours, at least, it was Hagerstown that was on his mind.

"It's really sort of crazy, isn't it?" he said later with a wide grin. "I suppose it could be seen as a dangerous sort of thing to do. Say it doesn't work. I'll not have done myself a bit of good. But frankly that didn't even enter my mind when I decided to do it."

He tried to explain to the assembled guests why the man who left the London Symphony Orchestra in 1968 to achieve great fame and fortune as a touring, and prodigiously recording, virtuoso now chooses to rejoin an orchestra, albeit for a few weeks each year, at the Hagerstown level. He cited several reasons. One was "an involvement in this area," where for years he has been bringing his horns to be repaired by one of the world's master craftsmen of the horn, Walter Lawson of nearby Boonesboro. Another was his fascination with "orchestra-building," going back to his days with the developing London Symphony.

But in an interview later it became clear that Barry Tuckwell, at 51, has been smitten with the same bug that is reaching epidemic level among international instrumental virtuosos as they reach middle age--the urge to conduct. Rostropovich is going about it in the biggest way. But there are also Rampal, Ashkenazy and even Stern on a few occasions.

Tuckwell noted that as an artist you can do only so much with the solo horn repertory. For instance, he is dropping out after many years as horn soloist with the Chamber Music Society of Lincoln Center, because "chamber music is not exactly where it's at for the horn, and also I'm tired of playing um-pah-pah." The music for solo horn and orchestra is limited as well, though the best of it, like the four Mozart concertos, is in enormous demand.

But much of the best horn music is back where Tuckwell left it in 1968, in the orchestra. No one can afford Tuckwell's kind of fees to have him come and play first horn for their Brahms or Mahler or Bruckner symphonies, so he sees his point of access now as a conduc- tor.

"It's my way of getting back to this magnificent repertory. I've been guest conducting already and will soon be doing more. At first as a soloist I didn't miss the orchestral repertory, but now I do, so this is a natural, logical turn of events.

"After 1968 I had physically left the London orchestra," he recalled as he sat back on a sofa in a Georgian gallery in Hagerstown's handsome little museum. "But I had always assumed that I would combine the two, the solo and playing in the orchestra. It didn't work. There is a different style to playing a continuous solo. And emotionally the soloist in the orchestra is still subject to the conductor's control and his interpretation, and I found it inhibiting."

How, he was asked, would conductor Tuckwell plan to direct an orchestra in a major French horn solo?

"I'd say it would depend on the horn player," he replied. "I'd say the lesser the player the more I'd have to have my way. That would be the case of a strong conductor with a weak player. The best balance between player and conductor would really be 50-50, wouldn't it?"

Tuckwell has played in the orchestra for almost all the major conductors of his time. What is his idea of a strong conductor?

"The conductor has ultimate responsibility for an interpretation. So the conductor must know how to use the players. I think the ultimate organizer . . . that I have encountered was Leopold Stokowski. He handled the orchestra like a jockey handles a horse, knowing exactly when to let the horse have his head, and when to control the horse."

He recalled his early encounters with Stokowski, when the conductor was in his 80s and 90s. "You can't imagine how frail he was, and sometimes he was in many ways not himself. I think in fact that there may have been times when he was up there before us when he didn't even know where he was, but it didn't affect his conducting. No one else had his ear for texture and orchestral sound. I guess I was expecting something quite different, something quite flamboyant, after all the publicity from 'Fantasia,' and the gossip about his life and the doubts some expressed about his taste. I was astonished at his economy of gesture. I don't know what it was that he had."

Is there anyone around today who can match those qualities? "Bernstein," he replied instantly, and then after reflection added, "That's two. And maybe that's it. Stokowski and Bernstein are the kind of men who, when they walk into a room, even those who know nothing about music know that they are somebody special.

"Orchestra players know that you don't fool around with men like that. Players like to be setting little traps, not always in a malicious way. They may be asked to play louder, and not do it, just to see how much the conductor really wanted it that way. If these minor instances of disobedience go unchallenged, the conductor will have lost a bit of the respect that is necessary for a fine performance. You can tell this in a conductor in about 15 minutes, maximum! It's a kind of gestation period. After that I have seen a perceptible if invisible curtain go up between conductor and players. And the better the players the more thay are simply bored. And there goes the performance."

But aren't these lofty thoughts a little remote from the practical situation in Hagerstown, where repeated efforts to get a volunteer orchestra going have failed?

"Not at all," asserted Tuckwell. "It is quite germane. We are going to have a professional orchestra where the previous efforts have been volunteer. And every time such an orchestra plays it can't help but be developing. In a sense, every concert is a rehearsal for the next.

"It was true of us at the London Symphony. When I started with them the orchestra wasn't that good, but by the time I left it was one of the top orchestras of the world Tuckwell was the LSO's chairman for seven years . It happened as the players got a better sense of how to rely on each other.

"Many of the most important details of orchestral playing have to be done by ear. Here in Hagerstown I get a chance to be in on it from the beginning. We've been having auditions and I'm amazed at the number of good performers we have right here in the town."

Tuckwell plans to commit himself to the new orchestra for six weeks a year (it will be four the first season, which begins next Nov. 13). At present the ensemble, which will have a base of about 50 players, remains in an embryonic stage. There is no budget, and no money. The orchestra will have to raise $100,000 for the first season, and it needs $10,000 for an orchestra shell to be built in Hagerstown's recently restored Maryland Theater. Players still must be picked.

None of this fazes Tuckwell. "My whole approach," he said, "is that there is just no question but that it's going to be an enormous success."