The first time that developer David Blumenfeld proposed an apartment house for the strip of land behind the Rodin Museum, he was practically laughed out of the Art Commission. His renderings showed a bland six-story building rising up like a wave behind the tiny classical temple, ready to swallow the Paul Cret masterpiece in its glassy maw.

What a difference a few months can make. When Blumenfeld returned to the commission last week with a revised design for 2100 Hamilton by Barton Partners, he was somehow able to talk members into approving the concept. It surely wasn't because the aesthetics had improved.

The building is now nearly twice as tall as before - a tsunami instead of a wave - and its angular blue-glass facade makes it look like the stunted cousin of the Cira office tower. The long slab of a building would create a new Chinese Wall behind the little Rodin.

Won't someone in City Hall please come to the rescue of this Philadelphia treasure?

Despite its diminutive size, the Rodin exerts a big presence on the Parkway. It packs in an extraordinary collection of the sculptor's muscular artworks. Cret's design was inspired by Renaissance villas, and like those country retreats, it is surrounded by a formal landscaped garden, created by Parkway designer Jacques Greber. A visit to the Rodin is like slipping off on a mini-vacation in Europe. It's a major tourist draw.

Blumenfeld's proposal would dramatically alter the experience by cutting off the northern third of the block. The Rodin would no longer be a building in space. It would appear visually jammed on a smaller, less serene, less countrified site.

In an ideal world, the Rodin would never have to share its block with another building. But because of an odd quirk in Philadelphia real estate history, the strip of land along Hamilton Street was never incorporated into the Parkway's green corridor, now part of Fairmount Park.

That's probably because no one expected that anyone would build on such a sliver of a parcel. Then about a decade ago, SEPTA sold off the section of the railroad right-of-way in the trench behind the museum. When you put the two pieces together and build a deck over the trench, you end up with more than half an acre, plenty big enough for an apartment house.

Of course, that doesn't mean the city has to be the project's enabler. It requires a raft of permits before it can be built, including a substantial zoning upgrade, from a rowhouse category to CMX-4, which allows dense, multistory development. Yet the Nutter administration, which increasingly takes an anything-goes attitude to development, seems intent on encouraging Blumenfeld. The Parks and Recreation department has already given the project a thumbs-up.

For a while, it looked as if Blumenfeld's project might derail at the Art Commission, which shares custody with parks and rec for the Parkway's cultural buildings. After rejecting the project this summer, its members agreed to work with the architects at Barton to come up with a more palatable alternative. It's hard to believe they think this version is better.

The complaint about Barton's initial scheme was that the apartments were too close to the museum. They've pushed them farther away - 87 feet from the back door. To accommodate all 120 rental units, they had to raise the building's height to 11 stories.

Clearly, they've made an effort to sculpt a more interesting form. It's just the wrong effort. The building's new angular shape - meant to follow the paths of Hamilton Street and Pennsylvania Avenue - only adds to the impression that it belongs in a suburban office park.

Barton also has filled in missing architectural details. Now we know they are planning an asymmetrically patterned, glass facade on a masonry base. By promising to clad the building in glass - probably blue, but perhaps gray - they were able to convince the commission that 2100 Hamilton is a "background building" that would appear to dissolve behind the Rodin.

The idea that glass magically renders buildings invisible is a persistent myth. It's true that glass can make a building feel ethereal and light, especially when it's draped like taut fabric. But that doesn't make glass buildings any less substantial or noticeable. The sharp contrast with the limestone museum will only accentuate its presence, especially because the facade of 2100 Hamilton spans most of the block.

There are certainly other big buildings nearby, including the sprawling Rodin Square development now going up on the north side of Hamilton Street. While it's a bigger building, the nine-story towers are arranged so their narrowest profile faces the museum. It doesn't "present a massive wall," observed Timothy Rub, director of the Philadelphia Museum of Art, which owns the Rodin Museum and is concerned about Blumenfeld's development.

You also have to wonder whether Blumenfeld really intends to complete the project after he gets his zoning change, which would significantly increase the site's value. Despite the building's high-end neighbors - the Rodin and the Barnes Foundation - he is pitching 2100 Hamilton as budget rentals. According to his architect, Seth Shapiro, he's even considering including micro-units. A slim, luxury-condo tower, pushed to the northeast corner of the site, would be more appropriate - and do much less harm to the Rodin.

Blumenfeld still needs to win over a roster of players to secure the zoning change, including the Logan Square Neighborhood Association, city planners, and, most important, Council President Darrell L. Clarke, whose district includes the Rodin. The change requires a Council bill.

Some say the city can't control the aesthetics of its buildings, but a zoning bill is a big ask. Now is the time for the city to use its leverage, before the site is upzoned.

There is so much construction in Philadelphia today, yet so little is of lasting architectural value. Here we have one of the city's - the country's - great buildings. Let's not sacrifice it for just another mediocre mid-rise.