Hieronymus Bosch. Rembrandt. Jan Van Eyck. Fra Angelico. Edouard Manet. Titian. Poussin. Whistler. El Greco. And many, many more.

It sounds like a a starry blockbuster loan exhibition. But the Philadelphia Museum of Art's "Old Masters Now: Celebrating the Johnson Collection," which will open Friday after several days of members' previews, has not come together that way. All its works have been under the Art Museum's roof, though not always on display, since 1933.

They belong not to the museum but to the City of Philadelphia, to which Johnson bequeathed them 100 years ago.  If you know the museum, you will recognize many of them as familiar treasures. The point of this lively show is that even works we think we know can yield surprises. As curators and conservationists look at and work with art, they have discovered much about how these pieces were first seen, how subsequent generations have altered them, and how even their chemical composition can change the way they look now. In one case, a skeleton was discovered right in the center of a canvas.

For decades, the collection was shown in galleries of its own, but after a 1989 court ruling, the Art Museum was allowed to integrate the works into its own galleries. This was a transformative moment for the museum, a colossal improvement over a layout that had placed closely related works far apart and required visitors to traverse art history twice.

No sensible person would suggest going back to the old segregated arrangement. But by mounting the show, the museum is at least acknowledging the desire of John G. Johnson (1841-1917), the eminent Philadelphia lawyer who amassed the collection, that it should have its own identity. We know that the Johnson Collection — unlike, for example, Henry Clay Frick's collection in New York — is not strong enough to stand alone, but at least occasionally it can stand apart.

Who was that mustached man?

In such a show, the collector is an important part of the story. By all accounts, Johnson was a very private man. In a posthumous painting in the show, his mustache appears a wonder of the Gilded Age — so large and long you are unable to see his mouth. Though his obituaries declared him the most eminent attorney in the English-speaking world, his life was neither social nor very political. He devoted his time to legal matters — often representing the huge corporate and financial interests that emerged during his lifetime — and acquiring a distinguished collection of art.

The New York Times declared in his obituary that Johnson "was probably less known to the public in proportion to his importance than any other man in the United States. It also noted that he twice turned down appointments to the United States Supreme Court, and he earned the largest legal fee, $500,000, ever paid for a single case to date.

The show includes one photograph of his house, near Broad and Lombard Streets, absolutely overwhelmed with art and puffy Victorian decor. It turns out that this picture was taken before he began his really extensive collection. Later, he moved to a larger house next door, which visitors reported was packed with paintings. They were hung on all walls and even the ceiling. Visitors reported that they were even leaning against the bathtub.

On the day before he died, he was not feeling well but still acquired two more paintings, bringing the total to 1,330 paintings and sculptures. He left them all to the city, and pointedly not to the institution that became the Philadelphia Museum of Art. Yet, because of a poorly drawn will and the city's financial strains during the Great Depression, the Art Museum became home to Johnson's art.

A lawyer’s eye for detail

The first portion of the show is devoted to Johnson as collector, though even here, he is elusive. He won great praise for his eye, but one gets little sense of his vision. Perhaps the attention to detail that he showed in his legal work carried over to his collecting. Some of the greatest works he bought are quite small, made either as devotional images or as small fragments of larger works. Jan van Eyck's devotional image of St. Francis of Assisi receiving the stigmata may be the greatest work in the collection. It's less than six inches square and contains a universe.

Most of the show consists of case studies of works that raise questions of interpretation, attribution, or conservation.

For example, one wall shows all 10 paintings that Johnson believed were the work of Bosch, challenging visitors to look carefully to figure out which one of the 10 is the real thing. This is a useful and interesting exercise, though it's easy to cheat and pick out the one that has been most carefully conserved.

In the case of Rogier van der Weyden's The Crucifixion, with the Virgin and St. John the Evangelist Mourning (ca. 1460), the question is how were these monumental panels first shown. A close examination of the wood shows holes that produced a conjectured configuration that helps explain how the work was seen. Similarly, a fragment of a Fra Angelico painting of Francis of Assisi was removed from a frame and allowed to stand as a silhouette, in the spirit of the way it was first seen.

Descent from the Cross (1518-20) by Joos van Cleve was long seen simply as a copy of a famous van der Weyden and kept in storage. But now, after two years of restoration, curators are looking at the way van Cleve varied, and in some ways enriched the composition that was his inspiration.

Titian is indelibly associated with the color red, but the cloak in the portrait of Archbishop Filippo Archinto (ca. 1558) looks muddy. That's because it was supposed to be purple but the combination of pigments and chemicals Titian used have degraded. Understanding the chemistry of the piece still does not explain the mysterious, half-veiled picture Titian made, though beneath part of the veil, a hint of the original color remains.

The Last Drop (ca. 1639) by Judith Leyster looked like a pleasantly boisterous Dutch drinking scene when Johnson bought it, but about 40 years ago, a scholar found an old copy of the work that was very different. By removing some later overpainting, conservationists discovered a rather horrifying skeleton holding a skull at the center of the picture.

In Pieter Janz. Saerendam's Interior of Saint Bavo, Haarlem (1631) the missing figures were hiding in plain sight. Though it was long thought that the people in the painting were simply generic and put in to give a sense of scale to this magnificent architectural interior, they turn out to be foreign royalty. That gives a whole different meaning to the picture.

"Old Masters Now" contains many pictures you have probably seen before. But its message is that it is worth looking again and again and again.

Old Masters Now: Celebrating the Johnson Collection

Hours: Tuesday-Sunday, 10 a.m.-5:00 p.m. (evening hours
Wednesday & Friday until 8:45 p.m.) Closed Monday.

Tickets: $20 ($18 in advance online), $18 seniors, $14 youth 13-18 and students with ID, free for children under 13.

Information: 215-763-8100 or philamuseum.org