At the end of his "Kubla Khan," after the gorgeous, exotic, opium-tinged vision has faded and he is trying to recall it, Samuel Taylor Coleridge zaps the reader with a double-barreled image, alliterating heavily with two strong words in the key of D: A damsel with a dulcimer In a vision once I saw . . .

Now, we all know what a damsel is, though you don't see as many of them around as you used to. But for years I have been wanting to ask Coleridge about that dulcimer: Did he mean an instrument that you hold on your lap, one that is shaped a bit like the hull of an old sailing vessel and has three strings that are fretted and plucked? Or did he mean the instrument that is usually played with a pair of hammers, a cousin of the Hungarian cymbalom and the Middle Eastern santouri (santour, santir) which was the instrument played by Zorba the Greek? If he really saw it, as he says, he should be able to tell the difference. The hammer dulcimer (which is also a cousin of the zither and the autoharp) has two or three dozen tuned strings stretched across a small, box-like frame, and when you hear one in the hands of a really good player, it can produce music of an almost orchestral richness.

Whichever dulcimer Coleridge saw as he was coming out of that pleasure-dome trip, he was invoking an instrument that has played a mighty role in the American folk tradition but one that has been somewhat neglected on records. The three-string dulcimer (which sometimes has four strings) is designed to play most naturally in the old-style musical modes that predate the classical major and minor keys, and has a particularly strong tradition in the Appalachian region, where those modes and some very old songs composed in them are still a living musical language. The hammer dulcimer was widely distributed through this country from colonial times until the beginning of this century (it was sold, for example, through the Sears, Roebuck catalogue), but it almost disappeared in the 1920s and did not make a comeback until the folk-music revival of the '60s, when it began to be heard at the Newport Folk Festival and elsewhere.

Both instruments are a pure delight to those who like to hear familiar tunes with an unfamiliar sound, and as it happens a prime source of recorded music on both instruments is a small, Washington-based company, Troubadour Records (1346 Connecticut Ave. NW, 20036). Troubadour's entire catalogue (seven titles on hand and four or five more on the way) fits on two sides of a typed page, but the four records I have heard make the company sound like a giant in its specialized field.

The two hammer dulcimer records are "Trapezoid" (Troubadour TR-1), played by the Trapezoid hammered dulcimer consort, and "American Hammer Dulcimer" (TR-6), played by members of the Original Hammer Dulcimer Club of Michigan. The approach is purely instrumental, with other folk instruments joining the dulcimers, and the material mainly of the almost-folk variety. The Trapezoid group has dulcimers in various sizes from bass to treble (like the stringed instruments in a classical string quartet), and its repertoire consists mainly of square-dance-style tunes, old and new, brilliantly played. The best-known number on the record is probably "British Grenadiers," and the effect of the playing, as one of the players remarks in the liner notes, is something like being inside a giant music box.

The Michigan group on TR-6 plays marches and hymns and old popular songs that stopped being commercial long ago. Best-known are such numbers as "Buffalo Gals" and "Twelfth Street Rag," but you can also hear "Irish Washerwoman," "Church in the Wildwood," "Great Speckled Bird," "San Antonio Rose" and "In the Garden." Those who remember these tunes from childhood (as I do) will find this collection marvelously nostalgic. For others, it may be a discovery.

The material is more purely folk -- Appalachian and the old English songs that have cousins in Appalachia -- on two albums featuring the fretted dulcimer: "Dulcimer: Old Time and Traditional Music" (TR-2) and "More Dulcimer" (TR-3). The featured performer on dulcimer is Ralph Lee Smith, who also does vocals, with Mary Louise Hollowell on TR-2 and Pat Kuchwara (an extraordinarily fine voice) on TR-3. Putting it as simply as possible: Anyone who is interested in American folk music should hear these records. I find the material slightly more interesting on the first record, which includes a high proportion of old English music, the performance somewhat more compelling on the second record, largely because of Kuchwara's voice. But both are fascinating and musically superb.

In addition to the content and the performance, Troubadour Records have an added attraction: an eight-page booklet written to accompany each record (except for "Trapezoid," which was the company's first production), giving information on the instruments, the music and the performers. This sort of annotation should be standard with any recording of serious music, and it is particularly helpful with folk material, which does not come with the kind of lavish documentation and commentary in books that is available for most classical music.