At midnight on the eve of the winter solstice, local sculptor Rockne Krebs put the last kink into his current mind-bender: A blue-green laser light, reverberating through a gallery at the Corcoran, bouncing off mirrors and back upon itself in a sort of cat's cradle.

"It's one of the most beautiful lines the cially likes the color.

Seeing "Crystal Oasis for the Winter Solstice," as the light sculpture is called, is the best way to understand the current exhibit of Rockne Krebs' drawings, which is also at the Corcoran. Personalized with the artist's reveries, reminiscences and political reactions, the drawings are studies for his light sculptures. They're also works of art themselves.

Harnessing sunlight, often using argon lasers to produce a lovely green, his plans appear absurdly impossible. One Krebs scheme that has not been realized is a "super-duper lightning sucker" that would collect electrical energy from the sky. Another is a rainbow tree, that, though seeming fanciful, is rather simply described by Krebs as "a plumbed tree with a light sensor that will turn on the water when the sun is in an angle that will produce rainbows."

These he views as more or less possibilities, unlike "One Sun II," where the instructions are to "make a pyramid of the sun truncated by a cloud." For this, Krebs explains, "I would have to be totally lucky. It would require making perforations in clouds."

For every great artist there's a level where imagination exceeds realization. Krebs' threshold is just a little higher than most. His mad-artist successes far outnumber his excesses: Sunlight sculpture by day and laser by night illuminate the ceiling of Atlanta's Omni; Baltimore's getting "Vine- covered Passerelles," green neon vines on a hi-tech pedestrian bridge; and, arching over the Miami River, a high, narrow, elegant train bridge will soon be illuminated by two 300-foot spectrum-bands of light.

The plans for these can be seen in the show, along with drawings of some evanescent light sculptures made for Washington -- "Ra," "Irish Light," "The Source" and, now, "Crystal Oasis for the Winter Solstice."

Inside the gallery, a warm otherworldliness envelops the observer. In the center of the room, three stylized palm trees glitter in glass and metal.

"They're supposed to be sculptures and sunpieces first," explains Krebs, stopping by the gallery to survey his oasis. "Then they get to be palm trees if they bear sufficient resemblance."

The palm-tree sunpieces are designed to stand by a window and "paint" a room with sunlight. At the Corcoran, when hit by laser light, they disperse spectra onto the walls. If you stand by the wall wherever you see a spectrum and look back at its source, you can catch a glint of red, a spark of yellow or a wink of blue from a prism at the tip of a palm tree.

Krebs says an oasis is needed for the shortest day of the year, when light is at its ebb. And the light he puts into the night is green -- so much a part of the natural landscape by day. "I think people do have a longing for this color," he says.

As Krebs noted in penciled scrawl on his study for "The Source": "There remains much that can be said of the value we place on green, but certainly nothing I can say can compare with being there." ROCKNE KREBS: A RETROSPECTIVE OF DRAWINGS, 1965-1982 -- And "Crystal Oasis for the Winter Solstice," at the Corcoran Gallery through January 22.