Prices leveled off and buyers suddenly seemed to become more cautious Friday night at Sotheby's, as the subject of an ongoing round of auctions switched from contemporary to Impressionist and modern art.

Twenty-three of the 79 works offered commanded $1 million or more apiece, but 23 other lots failed to meet their reserve prices and were bought by the auction house.In the third successive night of a weeklong series, Sotheby's sold 56 pieces of Impressionist and modern art for a total of $79.5 million, a figure that barely reached the auction house's low estimate.

Sotheby's officials speculated that the disappointing results may indicate that there is too much Impressionist and modern art on the market in the current marathon of sales. "People have so many opportunities to buy, they may be exercising a high degree of selectivity," said David Nash, head of Sotheby's Impressionist and modern art department.

But John L. Marion, Sotheby's chairman and chief auctioneer, cautioned against an overly gloomy outlook. "We get jaded. I don't know about you, but I think $8.5 million is a lot of money."

Marion was referring the top price in the sale, paid by an anonymous private collector for Pierre Auguste Renoir's Impressionist painting, "Bather." Emphasizing his point, he noted that the same painting (depicting a woman on a river bank drying her feet) sold at auction in 1985 for $1.85 million.

"Lowering the Curtain," an 1880 pastel by French Impressionist Edgar Degas brought the sale's second highest price of $7.98 million. An unidentified bidder purchased the richly colored interpretation of ballet dancers at the close of a performance.

A second Renoir, "Spring or the Conversation," brought $4.8 million from an anonymous private collector. A bright yellow and blue painting of a field worker by Vincent van Gogh sold for $3.4 million.