Last January, Daniel Terra, President Reagan's ambassador at large for cultural affairs, quietly bought five 19th-century American paintings for his museum in Evanston, Ill. Total cost: $2.5 million.

It was one of the biggest private transactions of the year in one of the hottest markets of the decade: Luminist paintings--glowing, transcendental images celebrating the American landscape.

Terra says he has "no idea" who owned the paintings.

"The Lano collection," he says with a shrug.

And what's that?

"I don't know," chuckles the silver-haired industrialist and finance chairman of Reagan's 1980 presidential campaign.

"And I really didn't care. When you're on the scent of important paintings like these, you don't ask questions that might queer the deal."

While big art deals--like all big-money deals--are often negotiated out of the public glare, Terra's purchase is extraordinary not only for its price, but for the network of behind-the-scenes art activity it brings to light in Washington. Even the handful of art world insiders who have heard of the Lano collection call it mysterious, "to put it mildly," says one curator.

Few outside the Lano circle--including Terra--appear to know all the facts. But it has been learned that the collection, which has kept a low profile for nearly a decade, still includes 60 to 70 paintings by 19th-century American artists, some of them important, even though the acknowledged best were sold to Terra.

It also turns out that the collection has no single owner named Mr. Lano, or Countess Lano, but that the name is an amalgam taken from the names of two venture capitalists and real estate developers: Raymond A. Lamontagne of New York City and Connecticut and Alan R. Novak of Washington.

Friends since Yale Law School, Novak and Lamontagne have been involved in several joint real estate and business deals, including the Tregaron Limited Partnership. Their highly controversial proposal to build 120 luxury town houses on the old Tregaron estate in Northwest Washington was unanimously rejected by the D.C. Zoning Commission in January.

Reached in Aspen, Colo., Novak--who served in two posts in the Johnson administration and was an 11th-hour Carter appointee to the Commission of Fine Arts (on which he still serves)--confirmed that he and Lamontagne are two of three principals in Lano partnership and said the paintings hang in "various homes in various cities."

"It is very private and includes only three close friends," said Novak. He would not reveal the name of the third Lano partner "for security reasons," but identified him as another "investor and businessman."

Lamontagne, an early staff member of the Peace Corps and an adviser to the late philanthropist John D. Rockefeller III, is now president of Encore Co. Inc., an investment banking firm. He says "Lano was a partnership formed to acquire paintings, and Al and I were general partners. But this was much more Al's doing than mine. I was the passive partner." He would not name the third limited partner, whom he described as being the major investor.

Novak says the collection, under the name Lano Arts, began in 1974-75. "We collected very actively for six or seven years until the market got so pricey after the National Gallery's 1980 Luminist show titled "American Light" that it was hard to improve our collection without the expenditure of huge sums." Since then, Novak says, Lano has been inactive, except for the sales, which he insists are not a signal that the whole collection is being sold.

If its owners never sell another painting, the Lano Arts will have been a profitable enterprise. No one would reveal the present value of the collection--or the profit made in the Terra deal--but in all likelihood, according to experts, the $2.5 million paid by Terra exceeds the original cost of the entire Lano collection.

"Except for the spectacular major names, it is conceivable that Lano or any other collector could have bought 60 paintings for that price if one started close to 10 years ago," says the National Gallery's deputy director, John Wilmerding. Luminists in the Shadows

Though Terra describes the deal as "a bit of a cloak-and-dagger operation," he admits he was not wholly in the dark as to what was in the Lano lode. Occasional paintings labeled "Lent by the Lano Collection" had been turning up in 19th-century American art exhibitions--including one at his own museum--and set his acquisitive heart thumping. It began pounding after the two paintings Terra wanted most and eventually got--Fitz Hugh Lane's "Brace's Rock, Brace's Cove" (1864) and Sanford R. Gifford's masterpiece, "Twilight on Hunter Mountain" (1866)--were included in the 1980 "American Light" exhibition, the National Gallery's landmark reassessment of Luminist art organized by Wilmerding. Lano loaned a total of five paintings to that show.

According to Terra, the purchase got under way last October when a New York lawyer called to ask if he'd be "interested in exploring the possibility of buying some paintings from the Lano collection."

"He said I had to agree that he would not be identified, and I agreed."

Arrangements were made for Terra to see the collection, and he was asked to go to New York. He said he was taken to two locations: an apartment on Manhattan's Upper East Side and a country house about an hour outside the city. Terra estimates that he saw 50 to 70 paintings that day, all by 19th-century Americans.

He says he did not know the man who picked him up at the Waldorf and drove him to see the paintings. He also says he never met the residents of the homes he visited and still does not know their names. His check was made out to--and delivered to--the New York lawyer.

"What I did know," says Terra, "is that Barbara Novak was the adviser to the Lano collection, and of course that gave it credibility beyond reproach."

Barbara Novak, the highly respected head of the art history department at Barnard College, is one of the world's leading scholars on 19th-century American art and has written two major books and several articles on the subject. She is also Alan Novak's sister.

The deal was struck in January, after months of negotiations about price and about which paintings could be purchased.

"They really didn't want to part with all their best things, but that's what I got," beams Terra.

The final package included a second oil by Lane titled "Dream Painting" (1862), William Stanley Haseltine's "Rocks at Nahant, Massachusetts," (1864) and Frederick Church's "Our Banner in the Sky" (1861), the only non-Luminist work.

The paintings are now at the Terra Museum of American Art, which zoomed out of oblivion last year when Terra bought "The Louvre" by inventor Samuel F.B. Morse for $3.25 million--the highest price ever paid for an American painting. Protecting the Wealth

Further information about Lano--including where the collection is stored--remains a mystery.

And that's how the owners want it, according to Patricia Madelyn Dey-Smith, an appropriately tight-lipped and nearly invisible Washington private dealer who calls herself Arcadia Inc. and is Lano Arts' curator and sole dealer.

Dey-Smith, who prefers to be called Pat Smith, says the Lano collection constitutes only a small part of her jet-set business, which specializes in 19th-century American art and deals worldwide. It may be one of the more lucrative art dealerships in town.

"Not just in town," she says. An interview was interrupted by calls from Saudi Arabia and Houston.

"I can't tell you anything about the Lano collection--who owns it, where it is or how many pieces. I can only tell you it is the finest quality," says Smith.

"Nobody has seen the Lano collection, knows what's in it or where it is except scholars," says Smith. "Nobody else has access. It is very private and very quiet, and there is no information available."

Described as "a wheeler-dealer" by friends--and by himself--Novak says, "I don't think there's any mystery. We've always kept the Lano collection very private because paintings have a way of attracting people--it's a security problem."

Novak says that after an unsuccessful break-in at his Georgetown home last year, all major Lano pictures were sent to New York.

"There are many such collections that nobody knows about," says Pat Smith. "The Lano is not unique. Secrecy is a way of life for many people of wealth who don't want anyone to know what they have. Security is an especially serious problem for those who have several homes sprinkled around, especially when you read that institutions with armed guards in every room still have major thefts.

"People are really quite paranoid about it, and I think they're justified," she says.

Alan Novak says it was such security considerations that--at least in part--prompted the Terra sale. "We felt these paintings had become too valuable to keep in the house. We also felt they had become important art historically and should be in a public museum. The paintings were sold now because Dan Terra is acquiring now, and we think he's building a great collection . . .

"Besides, there's nothing wrong with getting a good sale on them . . . My sister is the one who took a vow of poverty, not me." The Sister, the Scholar

Alan Novak says his sister Barbara "owns no part of the Lano collection and never will," and that she has benefited in no way whatever from the $2.5 million Terra sale. "She's always insisted that those were the rules of her profession, and I regret it," said Novak. "After all, she's my sister, and an academic, and I'd like to give her a piece of the action."

Reached in Dublin, Barbara Novak said she had helped her brother form Lano's "highly personal" collection, and that she was proud of the results. "I've served as a paid scholar-adviser to Lano, just as E.P. Richardson did for the Rockefellers, and John Rewald did for the Whitneys and innumerable other scholars have done throughout history."

"Except she's not doing it for some rich man, but for her brother," says Alan Novak. Collectors or Investors?

The question of whether Lano was formed as a personal collection or as a profit-seeking investment group is, in part, what makes scholars and dealers so curious about it. Even the most serious collectors sell occasionally to upgrade their holdings or to pay for important new acquisitions. The difference is that collectors are treated with deference by museums (who hope a gift might come their way) and investors are treated with contempt because they jack up prices and make museum purchases more difficult.

Barbara Novak says investment was never Lano's goal. But businessman Novak says: "When we started, I'm not sure we knew what our intent was. We thought the American art market would be a good investment; whenever you put capital to work, you have to assume some investment aspect.

"But as it developed, so did the desire to make it a great collection. We had plans to circulate it some day, and have Barbara write a catalogue. Maybe we still will."

The sale to Terra is not the first from the Lano collection. Martin Johnson Heade's "Sunrise Over the Marshes," lent by Lano to the White House from 1978 to 1980, was sold through Arcadia soon after acquiring its presidential gloss.

"We had three or four other Heades in the collection and sold that one to a friend who had always wanted it," explains Smith. "There were also a few other little pictures that weren't of sufficient quality, or that didn't wear well, that have also been sold." But she and Alan Novak both say that no more than 10 percent of the Lano collection has ever been sold.

Some had begun to think otherwise.

Wilmerding says the Lano sales took him by surprise. "I just thought it was a collector's group. Now I think it's an investor's group. Since they've sold some of their key things, it's now clear that they're not putting them together to form a museum collection, which is what I naively thought."

"This transaction is absolutely not a signal that the collection is up for sale," says Barbara Novak. "There's not a word of truth in that. The paintings sold to Terra were most reluctantly parted with."

Though the paintings that changed hands were tremendously important, she says, "there is a great deal left to enjoy--roughly 60 paintings." Among the highlights, she lists a Winslow Homer watercolor titled "Orange Trees and Gate," a Martin Johnson Heade "Hummingbird," a "lovely" Bierstadt "Horseback Riders" and a watercolor by William Trost Richards. Good Buys

The "embarrassingly small sum" Novak says he paid his sister to advise him was clearly a good investment in at least one extraordinary way. "Brace's Rock, Brace's Cove" by Fitz Hugh Lane--one of the two most valuable paintings sold to Terra--would probably never have come to Lano except for the fact that when it was discovered in 1978 in a house in Maine, the owners took the painting to Barbara Novak for identification. She identified it for them and advised Lano to buy it. And they did--for considerably less than $200,000.

"Twilight on Hunter Mountain" by Sanford Gifford, the most valuable work to change hands, was purchased by Lano in the late '70s for only $90,000.

Whatever Lano's profit on those two paintings--and the entire Terra deal--it was substantial.

Yet some say Terra got a bargain.

And others say he paid too much.

But Terra is ecstatic. "It was a good deal for both of us. They got their money right away, and I got the paintings I wanted."

Alan Novak is pragmatic: "I wish every collector could have a sister like Barbara." CAPTION: Picture 1, "Twilight on Hunter Mountain" by Stanford R. Gifford; Picture 2, Rocks at Nahant, Massachusetts," by William Stanley Haseitine; Picture 3, "Dream Painting" by Fitz Hugh Lane; Picture 4, "Brace's Rock, Brace's Cove" by Fitz Hugh Lane, on of the most valuable works in Lano's $2.5 million sale to Terra. Paintings photographed by Michael Tropea