Huntington Museum; EIGHTEENTH CENTURY OPULENCE

Botanists, bibliophiles, and art cognoscenti flock to this exclusive Los Angeles suburb to visit the Huntington Museum. But this unusual institution - which combines an art gallery, library of rare books and manuscripts, and botanical gardens - while well known to scholars, is still relatively unknown among tourists.

I might have missed the Huntington had it not been for a TWA flight attendant who recommended it as one of her favorite places to visit in the Los Angeles area; her descriptions of the beautiful gardens, especially the extraordinary Japanese Garden, were deserved, down to the last detail.

The Huntington is 12 miles northeast of downtown Los Angeles, in what was once the home and headquarters of transportation tycoon Henry E. Huntington and his wife, Arabella. Its 207 acres of cultivated and landscaped grounds, richly decorated gallery, and impressive library, reflected the personal tastes of the Huntingtons.

Mr. Huntington was fascinated by the influence of English and French intellectual history of the 18th century on American culture and traditions. Therefore he collected 18th-century English and French fine and decorative arts, and English-language manuscripts from various periods of history and schools of thought. Naturally, Mr. Huntington's areas of emphasis have become the Huntington's areas of specialization (although he insisted that his life and personality not be the focus of the institution and in fact there are very few references to him in any of the museum's brochures or information sheets.)

My first impression of the Huntington was of surprisingly vast expanses of lush green lawns, unexpected in this semiarid region bordering the desert. These lawns, surrounding a dozen specialized botanical gardens, are completely renewed each year, after the inevitable summer scorch. There is not one ''keep off'' sign posted in the 207 acres, which are instead punctuated with lovely stone and bronze statues and delicate gazebos.

Free guided tours of the gardens leave the main entrance at 1 p.m., Tuesdays through Saturdays; visitors may also wander through the grounds with rented tape tours or self-guide booklets. I headed across the lawns to the Desert Garden, a fascinating 12 acres densely planted with 2,500 species of cactuses and other fascinating desert plants, some astonishingly beautiful, others almost grotesque. Even the names are intriguing: pincushion cactus, elephant's foot, cow's horns, heart of flame, and others. This is the largest outdoor desert garden in the world.

The Rose Garden is an aromatic bower of subtle colors, with over 1,000 varieties of roses arranged in order of historical development, many of them actually developed at the Huntington, introduced there, or both. Roses are also planted in the Shakespeare Garden, an Elizabethan-style formal garden. Most of the blooms mentioned in Shakespeare's plays are included in the landscaping, as is an intricate pattern of circles and diamonds traced in clipped shrubs and herbs - a modern adaptation of the Elizabethan knot-garden. Nearby are dense plantings of camellias and azaleas.

The Herb Garden, arranged in groups for cooking, salads, confections, cosmetics, dyes, perfumes, and other uses, is the only garden where touching is permitted. Poking through these spices on the vine is a delightful educational experience.

The Subtropical Garden, Lily Ponds, and Australian and Palm Gardens are equally impressive, but my favorite was the Japanese Garden. The gateway is flanked by two large lion-dogs, used in ancient China to guard the entrance to an important residence. Throughout the garden's various sections are votive stones, lanterns, and pagodas - each a beautiful object with importance for Japanese culture and history. There are beautiful flowering plants and trees, Japanese red pine, azaleas, and camellias.

Within the Japanese Garden, in a shallow valley, is a beautiful reflecting pool, spanned by an arched vermilion bridge, known as a ''moon bridge,'' because the bridge's gentle arch and its reflection in the pool beneath form a perfect full moon, and because ''moon viewing'' from beneath the bridge was a popular evening entertainment for estate owners cruising in small boats on their private lakes. Also associated with the lake, near the moon bridge, is a small spirit shrine, a miniature house built as an offering for ''the spirit of the lake'' to bring good fortune to the owner.

A five-room Japanese house, typical of wealthy homes of the Meiji period (late 19th century), illustrates various ways Japanese houses are organized to allow for contemplation and enjoyment of nature, and the surrounding gardens. Next to the house is the Zen Garden, an open expanse of raked gravel representing a flowing stream. In Japanese culture, this type of garden provides symbols of nature that bring forth a meditative state of mind, in which the viewer uses his imagination to complete the scene in an exercise of insight and awareness. Two additional art forms, Sui-seki (or water stones) and bonsai (or miniature trees) are also well represented. Altogether, the Huntington's Japanese Garden is an exquisitely serene and timeless environment.

But the gardens are only one aspect of the Huntington. In the midst of this gorgeous greenery stands the Art Gallery in the main house, a large white two-story mansion, built during the years 1900-11 in a formal, sumptuous style; most interiors are modeled after English or French rooms of the 18th century.

Each room represents a different decade, illustrating the evolution of style during that period, with furnishings that were once used by aristocrats. Exquisite wall tapestries, designed by Boucher and woven at Beauvais, and ornately carved and gilded chairs and settees with tapesty covers woven at Gobelins, chandeliers, hallmarked pieces of silver, delicately patterned inlaid wooden desks and tables, Sevres porcelain and Wedgwood, crystal, carpets, a dozen clocks, and other objets d'art that beautified the elegant lives of the 18 th-century gentry; these decorate every room.

As I wandered through the mansion, I felt I had entered another era; only the silk-clad wigged courtiers were missing. But portraits of them are hung in the mansion's large drawing room, an ideal setting for the Huntington's exceptional collection of full-length 18th-century British portraits, including Gainsborough's ''Blue Boy,'' Lawrence's ''Pinkie,'' and ''Mrs. Sarah Siddons as the Tragic Muse,'' by Reynolds. The hall also displays English landscapes, notably Constable's ''View on the Stour Near Dedham.''

The library, the third aspect of the Huntington, is in a separate building. It is primarily a research institution, devoted to the study of British and American history and literature. It was founded by Mr. Huntington in 1919 and today contains a truly awesome collection of about 6 million items, including books, photographs, prints, and historical and other documents and manuscripts.

Many of the library's greatest treasures are shown in the main exhibition hall, a stately wood-paneled room with five tall hexagonal display cases. Twenty-eight other cases contain major works by British and American writers, statesmen, scientists, explorers, and builders of the American West.

The treasures in the five principal cases are: the Ellesmere manuscript of Chaucer's ''Canterbury Tales,'' dated 1410; the Gutenberg Bible, the first book printed with movable typeface in Europe, dated 1455; Shakespeare's first folio, printed in 1623, and a book of his sonnets published in 1606; Benjamin Franklin's ''Autobiography'' in manuscript and letters and other documents by Franklin, George Washington, and Thomas Jefferson; and Audubon's double folio of ''Birds of America,'' dated 1827-38.

Among other items on display during my visit, two left deep and lasting impressions on me. I was fascinated by Jack London's energetic and inspired scrawl as he penned the first page of ''To Build a Fire,'' his most famous story , and the beauty of William Blake's words and illustrations in ''Songs of Innocence and of Experience.''

The Blake manuscript is available in facsimile in the Huntington Bookshop, which offers a wide range of reproductions of artworks, books, prints, photographs, documents, old posters, and new titles of scholarly works based on research at the Huntington Library. Most of these items, published by the Huntington, are remarkably inexpensive.

For example, that reproduction of Blake's ''Songs of Innocence and of Experience,'' selected and introduced by James Thorpe and in full color, costs only $2.50. A 32-page paperbound facsimile of the 1800 edition of ''Cinderella: or the History of the Little Glass Slipper'' sells for $1. There are also photo slides of paintings, the gardens, and views of the estate, as well as potted plants of the more unusual varieties found in the gardens. The Bookshop also sells by catalog, available upon request from the Huntington, 1151 Oxford Road, San Marino, Calif. 91108.

The Huntington is open from 1 to 4:40 p.m., Tuesdays through Sundays. Admission is free; reservations are required on Sundays. For Sunday reservations , send a self-addressed, stamped envelope to Sunday Tickets, at the above address. For further information and transportation directions, phone (213) 449- 3901 or, from Los Angeles, 681-6601.