SOME YEARS ago Albert Fuller played a brilliant organ recital in Washington Cathedral. Afterwards, he said to some friends who were praising his playing. "That's it. I am never going to play the organ again. I am going to spend all my time on the harpsichord."

And that is exactly what he has done.

At Yale, where he went to study the harpsichord with Ralph Kirkpatrick, he won the Ditson Fellowship that sent him to Paris and the Bibliotheque Nationale. There he carried on further studies that helped make him today one of the world's top authorities not only on playing the harpsichord but on the many worlds the harpsichord inhabits.

"You have to decide what a harpsichord is," Fuller says when you ask him how and where a person starts to play one of those things that audiences jam around to stare at during the intermissions of baroque concerts.

"And that isn't very clear today. It still is not completely clear what an honest-to-God harpsichord is, since the literature covers 400 years, and since instruments vary from country to country, and from time to time during those years.

"And since the revival of the harpsichord, the concepts of understanding what history was, and what music was used for, have changed. And since harpsichords during the revival have changed so tremendously, it has not, at any time up until recently, been easy to say, "This is an instrument that a certain genius composer would have recognized as the thing he composed for.'"

"Was the purpose of the harpsichord different in Germany from the purpose of the harpsichord in France?" he was asked.

His answer was an emphatic "Yes," and he elaborated: "And in Italy particularly. Because note, there aren't any important harpsichord composers idiomatically in Italy between Frescobaldi and Domenico Scarlatti. The Italians aren't interested in solo keyboard music.

They're interested in fiddle and the voice.

"And so Italian harpsichords did not show either the visual or the mechanical and hence the physical technical changes over the hundreds of years that the French harpsichords did."

In view of Fuller's long experience in playing in Monteverdi operas, the next question seemed logical. "What did Mosteverdi see as the function of the harpsichord in his orchestra?" (He used two.)

"He tells us what he wants them to do," Fuller began. "He wants them to support the sequence of harmonic events that happen and to give the harmonic color over which the single singer makes the new style. When I first began to play in Monteverdi, it was perfectly clear that the singers did not know - from shinola about any of that.

"And they still don't know, because the voice is such a precious thing. It's your body, and to learn to sing another way is a very hard thing to do. As a harpsichordist, I can go to a museum and look at an old harpsichord. But you can't go and look at an old voice. It isn't there. And that's why it seems to me that the correct style of singing is the last thing we are going to get into in this technologically inspired style revival.

"It's clear now that what Monteverdi expected of singers resembles what we call cabaret style in singing today - an expressive feeling with virtuosic vocal approaches that are to be made up by the singer, with ornaments and things against the beat and with the beat and across the beat. And the harpsichord player should be like the good accompanist to the cabaret singer."

It was becoming clearer by the moment why Fuller's students at Yale and the Juilliard School find his teaching exciting. From Italian opera, the talk worked its way around to just what Italian music Wanda Landowska, the great 20th-century reviver of the harpsichord, had played.

"She played a lot of Scarlatti, of course," Fuller started out. "But the thing about Landowska is that she was a great - I am not able to call her a genius, I don't know, I have to get farther away, I'm too caught up in what she did - but she was a tremendous performer.

"Her harpsichord was an invention of the 20th century, her Pleyel. But it did not have a big sound. Our harpsichords sound twice as loud as hers did. And hers have a heavy action that was modeled on that of the piano. Because those things that Pleyel built were still under the assumption that art means getting better, rather than changing.

"That meant that whatever happened to the piano in the 19th century was appropriate. And if the harpsichord had only survived that century, it would be good for it to have that too. So thick frames, heavy metal frames, thick soundboards, heavy action were used in order to stiffen the action.

"But the great harpsichords were based on the fact that the harpsichord box was rather like a violin or a lute. The bottom was the resonant thing. It's a resonant box like a violin. The best French harpsichords, for example, in the 1720s through the '80s, sit on stands. But they are only touched on three different places. So the box is going like this all the time." (Here Fuller made a picturesque gesture with his hands, suggesting the vibration of the harpsichord.)

"It looks as if, in Louis XVI's time, that there are lots of legs - it looks as if it's sitting on a stand. But it only touches on just the three corners. Because you don't want to touch that box. And there's no ormolu on that box - it's only painting. There's no veneer. There's nothing! It would be like veneering a violin, putting on ormolu thing on a violin, you wouldn't dare do that.

"But you see, the 20th century builders began to remake the harpsichord as if what had happened to the piano was correct for them. Now that made the harpsichord sound like one kind of thing; and Landowska, being a genius performer, made music with it and made interpretations that grew directly out of what it wouldn't do and what it would do. She took what it did do and played it to the hilt.

"But," and here came the clincher, "it (meaning the 20th-century Pleyel harpsichord) doesn't resemble what Couperin or Bach or Scarlatti or anybody else in the period that produced harpsichord music had in any respect except for the fact that something plucked the strings!"

After a brief pause, Fuller went on. "I often use visual analogies. For instance, nowadays people use acrylic paint. Acrylic paint is a very special thing. And it can be used with a brush. But of course it does not respond to a brush the way that anything that Degas did respondend, or Veronese or anybody else. Its luminosity is completly different, the brush strokes don't end up the same, and the colors are completely different. So if we see a painting of Veronese, say, in Day-glo, for example, we're not seeing Veronese's brush strokes. And yet part of his genius is the result of the movement of his hand."

Something else basic needed an answer. "Why are there two keyboards on the harpsichord? Is it a matter of color or timbre?"

"That's a curious question," Fuller replied. "They is a mystery that relates to it, which we have not yet unlocked.The function of the two keyboards is an attempt to make the instrument more expressive, relative to other instruments.It came through a dissatisfaction with what you could get out of one keyboard, particularly when other instruments began to make their appearance in French chamber music.

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"You see, the harpsichord, as we understand it today, is related intimately with the lute and gamba. It's a keyboard lute, just like the gamba in fact is a bowed instrument, it is a bowed lute. The French always thought of gamba playing as 'plucking with the bow," which is not what the rest of Europe saw in gambes at all.

"But the Marais and the Forquerays and all the French were very unified in that; the lute and the gamba and the harpsichord for them were all varieties of similar esthetic approaches. The French were so tied to lute music and to extension of what lute music said because it relates to the French language. It doesn't flow in long waves like Italian. It's stopping and starting. There's a unity between French melodic devices, French expressivity and the music that relates to them.

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"Now this two-manual thing has been confused with organ music, but the facts do not warrant it. Every piece of French organ music tells you what to play it on. What registers to use. And three-quarters of the pieces are named by the registers you use: They're called Recit de trompette, and jeux d'anches, and so on. So intense is their preoccupation with color and the effect of the music that you write one way for this kind of stop and another way for that one.

"That's important. But there is nothing and no mention in any harpsichord piece about registration whatsoever until a published piece of Dandrieu about 1730, which is a hundred years after it all started. He says, in a piece about birds, 'If you can, please use only the petit octive. ' Meaning, please use the four-foot to make the bird sounds.

"In the early days of the harpsichord revival, we were very easily confused all the time: We thought that you could play this on this, and this on that. But the styles of writing for the two instruments are very different, though when you're untutored, the page looks the same. But in Bach, for instance, who was a genius, no page of organ music looks anything like a page of Bach harpsichord music."

At length the subject turned to certain technical problems: "What are the possibilities of varieties of touch on the harpsichord?"

Again the answer had its qualifier: "You have to say which harpsichord," Fuller began. "We will have to say as we today understand the classic harpsichord of the 17th or 18th century, and not the various instruments that have come down the pike in the 20th century. I would then need to talk about the harpsichord of Rameau and Couperin and Bach and Handel and Scarlatti - all of those people.

"First of all, the action is very light. The aim was to make the action as light as possible, to have the least amount of inertia or resistance between the movement of the finger and the end of the key and the plucking of the string. Psychologically, when you do this, you are in touch with the plucking of the string when you push the key of the best harpsichord actions. The key does not stop because it goes down; the key stops because the jack comes up to the top of the jack-rail, having plucked the string.

"The whole new harpsichord technique, the new old technique has to do with nothing that involves the shoulder or the arm for push, nothing of that at all. It has to do with moving the fingers over the keys and playing either from the key without lifting the finger from it, so that the plectrum comes up and touches the string and goes 'gnnnnnnnnnng,' or striking the key so that the plectrum slaps the string; and all the varieties between those two. What exists in the piano as a large variety of weights and pressures has, in the harpsichord, to be translated into silence and sound and what the French called, in the old days, le silence d'articulation.

"Without an instrument to explain it, it's very hard to verbalize. But none of this music was conceived of originally as being legato. Everything was separate, just as all syllables are separate. In playing the harpsichord, everything has to conceived of as 'Does it have a silence of space before it, and how long does it hang on, and does it cover up the next note?' And how to make the instrument have waves of plasticity that fit in with violin playing, say, but not our modern Russian violin playing, because Bach and Rameau didn't have that. They had another kind. And that has been another of the clues to harpsichord playing."