As architect of the Library of Congress' $81.5 million, six-year remodeling, Arthur Cotton Moore aims "to do it invisibly."

Fat chance! Anything as big and as hard to miss as the 13.7-acre Thomas Jefferson building of the Library of Congress, with its 104 miles of bookshelves, is bound to be controversial. Updating the library's magnificent 1897 building is like trying to update Shakespeare.

*After the White House and the Capitol, the library is probably the most-loved building in the capital, and perhaps the finest example of neoclassical architecture in the country.

Its renovation, minute compared with the Pentagon's planned $450-million addition, is still expensive and extensive. The restoration of the building's gilded and glorious ornamentation and installation of modern climate control, fire control, plumbing and electrical systems have begun with the award of a contract to Grunley-Walsh Construction Co. The bid was 10 percent below expectations. Some repair work and demolition has already been done. The Neptune Fountain is bubbling again and the West Terrace is no longer in danger of dumping its visitors.

The cost for art restoration alone will be somewhere near $1.5 million. Elliot Carroll of the Capitol architect's office said the library has more original artwork than any other government building. Restoration of the murals, for instance, will take just about every mural conservator in the country, he said.

The design for remodeling, restoration and refitting of the building won the 1981 historic preservation award by the American Institute of Architects' Washington Metropolitan Chapter.

But now that a model of the renovation project is on view in the library, and people are shifting around, out of the way of the workmen, questions arise as to just how invisible the remodeling will be.

* Most of the condemnations and congratulations swirl around the installation within the vast marble chambers of the first and second floors of six "gloriettes," or self-contained structures designed to make smaller rooms while blending with the library's neoclassic decor. On the second floor, they will be offices. On the first floor they will be offices, microfilm reading rooms, card catalogue rooms and research services space.

These fanciful temples -- Moore calls them "big pieces of furniture" -- will be sheathed in oak to match the library's existing woodwork. Their fac,ades will be what the Victorians called a "tasteful" arrangement of arches, columns, balustrades and an assortment of architectural doodads.

* In the great architectural tradition of doing nothing to a historical building that cannot be undone, Moore said, "The structures float. In the future, if people don't like them anymore, they can be removed, leaving no trace.

"The offices we're building won't touch the original building except at the floor. They are designed to keep people from trashing the inside of the library as they have over the years," Moore said.

* Most gloriettes will be 25 feet wide, 100 feet long and 9 feet 9 inches high. The ones on the first floor, less exalted space, will sit in halls only 18 feet 4 inches high. On the second floor, they will sit in ornate halls 35 feet wide and 220 feet long and about 30 feet high. So the elaborate original ceiling will be about 20 feet above the roof of the gloriettes. This will permit additional mezzanine-style offices atop the structures, though so far only one is scheduled, says Jim Trew, head of the library's building management. Five feet on each side will be left around the gloriettes for corridors so the structures won't block the library windows and so the office occupants won't be disturbed by traffic. The office structures will take up less than half of the length of the great halls.

All the electronic and mechanical services expected to be necessary for 21st-century offices will be built into the gloriettes, much of it enclosed in posts and beams, thus preserving the floor. The decorative urns on the tops of the doorways will deliver air.

* The structures will take the place of the two-story "tempo" shacks that allowed squatter offices to homestead in the great ornate galleries for years.

*The gloriettes won't be the first thing you'll see as you enter these halls. To reach the experts in the offices at the back of the curtains, you'll have to pass through a "court of honor."

Triumphant arches -- fitted with lights, flying flags and displaying posters, engravings or manuscripts on the hall's subject -- will guard the rows of books marching down the gallery.

Free-standing monumental bookstacks -- suggested by those Christopher Wren designed for Oxford in 1676 -- topped with busts of wise men, and emblazoned with wise sayings, will go in double waves down the hall.

Behind the bookcases, in the hidey-nooks, scholars can toil away in privacy. The richly decorated ceiling with its recessed rosettes (fitted with sprinklers) will be restored to its 1897 grandeur. In some areas, cabinets from the old card catalogue (superseded by electronic catalogues) will be refitted with computer terminals.

* All this is part of Librarian of Congress Daniel Boorstin's multimedia encyclopedia of knowledge. The three volumes will be made up of the principal library buildings: the Thomas Jefferson (the original building), the John Adams (built in 1938 and to be remodeled as part of the project) and the new James Madison (finished in 1980 at a cost of $123 million -- the third largest Washington government building).

* As Boorstin explained to Congress, "the great domed Main Reading Room will be the center of all reference activity -- the index to our encyclopedia." From there the seeker after knowledge will go to the specialized halls -- European civilization, for one -- where the books, posters, prints and other material will be available adjacent to the experts.

"This organization of our research staff and collections provides a level of personal service and access to our collections greatly superior to what the library has been able to offer," Boorstin told Congress.

*The whole middle block of the main West Pavilion, the principal public space with its great ceremonial staircase, Trew said, will be cleared of exhibits, except for places for people to sit and contemplate its glory. As Moore puts it, in the West Pavilion, the building itself is the exhibit. The northwest and southwest corner pavilions and galleries will be devoted to exhibitions of what Moore calls the library's "heavy-duty treasures": the contents of Lincoln's pockets at his death, the Gutenberg Bible, original music scores, Art Nouveau posters . . .

Moore's first design for the cases, library officials said, was not suitable for the mostly flat objects the library has. So Moore toured European libraries and now is proposing "Victorian jewelry cases on a grand scale."

The Main Reading Room will be restored and fitted with noiseless carpeting in a pattern derived from a plaster motif of leaves and a book. The room will be closed from June 1987 until June 1988.

Its great domed ceiling, Trew said, is in fine condition because it was restored 10 years ago. The arcades of the Main Reading Room's upper levels will be glassed in. Level 2, now in stacks, will become more desk space for scholars. Moore says from the third-level arcade visitors will have a "gee-whiz view of the reading room."

*Other changes are planned.

An orientation theater on the ground floor adjacent to First Street will be enlarged, with the present labyrinth of dry-wall partitions removed to show off the building's vaulted spaces. Public lockers will be nearby.

*A restaurant for tourists and staff will be built into the southwest cellar looking out through bay windows and expanding as an outdoor cafe' into the last remaining of the library's original four 21st-century offices will be built into the gloriettes, much of it enclosed in posts and beams, thus preserving the floor. The decorative urns on the tops of the doorways will deliver air.

* The structures will take the place of the two-story "tempo" shacks that allowed squatter offices to homestead in the great ornate galleries for years.

*The gloriettes won't be the first thing you'll see as you enter these halls. To reach the experts in the offices at the back of the curtains, you'll have to pass through a "court of honor."

Triumphant arches -- fitted with lights, flying flags and displaying posters, engravings or manuscripts on the hall's subject -- will guard the rows of books marching down the gallery.

Free-standing monumental bookstacks -- suggested by those Christopher Wren designed for Oxford in 1676 -- topped with busts of wise men, and emblazoned with wise sayings, will go in double waves down the hall.

Behind the bookcases, in the hidey-nooks, scholars can toil away in privacy. The richly decorated ceiling with its recessed rosettes (fitted with sprinklers) will be restored to its 1897 grandeur. In some areas, cabinets from the old card catalogue (superseded by electronic catalogues) will be refitted with computer terminals.

* All this is part of Librarian of Congress Daniel Boorstin's multimedia encyclopedia of knowledge. The three volumes will be made up of the principal library buildings: the Thomas Jefferson (the original building), the John Adams (built in 1938 and to be remodeled as part of the project) and the new James Madison (finished in 1980 at a cost of $123 million -- the third largest Washington government building).

As Boorstin explained to Congress, "the great domed Main Reading Room will be the center of all reference activity -- the index to our encyclopedia." From there the seeker after knowledge will go to the specialized halls -- European civilization, for one -- where the books, posters, prints and other material will be available adjacent to the experts.

"This organization of our research staff and collections provides a level of personal service and access to our collections greatly superior to what the library has been able to offer," Boorstin told Congress.

*The whole middle block of the main West Pavilion, the principal public space with its great ceremonial staircase, Trew said, will be cleared of exhibits, except for places for people to sit and contemplate its glory. As Moore puts it, in the West Pavilion, the building itself is the exhibit. The northwest and southwest corner pavilions and galleries will be devoted to exhibitions of what Moore calls the library's "heavy-duty treasures": the contents of Lincoln's pockets at his death, the Gutenberg Bible, original music scores, Art Nouveau posters . . .

Moore's first design for the cases, library officials said, was not suitable for the mostly flat objects the library has. So Moore toured European libraries and now is proposing "Victorian jewelry cases on a grand scale."

The Main Reading Room will be restored and fitted with noiseless carpeting in a pattern derived from a plaster motif of leaves and a book. The room will be closed from June 1987 until June 1988.

Its great domed ceiling, Trew said, is in fine condition because it was restored 10 years ago. The arcades of the Main Reading Room's upper levels will be glassed in. Level 2, now in stacks, will become more desk space for scholars. Moore says from the third-level arcade visitors will have a "gee-whiz view of the reading room."

*Other changes are planned.

An orientation theater on the ground floor adjacent to First Street will be enlarged, with the present labyrinth of dry-wall partitions removed to show off the building's vaulted spaces. Public lockers will be nearby.

*A restaurant for tourists and staff will be built into the southwest cellar looking out through bay windows and expanding as an outdoor cafe' into the last remaining of the library's original four courtyards.

* In the librarian's ceremonial office and the Congressional Reading Room, the balcony's arches, now boarded up with plywood, will be reopened.

Even the tunnel that connects the three buildings will receive its due -- a mural of book jackets.

While the refitting is the most visible part of the scheme, restoration and renovation will absorb most of the money. Moore points out that until recently, the only fireproofing the building had was the occasional fire extinguisher. Now chemical sprinklers have been installed in the bookstacks, with sprinklers in the rest of the library, new climate control systems, new plumbing, new lighting and miles of cables for electronic systems in the offing. Much cleaning, plastering and repainting will be done along the way.

*If Washington is fortunate, when the library's 100th anniversary comes up 11 years from now, the renovation will have produced a building combining the charm of the late 19th century with the technology of the early 21st.