How often do you get to sit in a work of art? How often do you get to escape into a room that feels absolutely, totally, unconditionally cut off from the world around you, to the point where you feel kind of lost after a while? If your answer is "never," then clearly you have not trekked to the northern end of the Nasher Sculpture Center, where James Turrell's so-called "skyscape" awaits even the most claustrophobic among you and promises something close to nirvana. Since we can no more describe the place than we can say how it makes us feel--chilly in summer, warm in winter, wonderful always--then let the Nasher's Web site do it for us; we will add only that it's the only site-specific creation at the joint, which is fast becoming our favorite local retreat. Says the site about Turrell's creations: They're "enclosed spaces--rooms or free-standing structures--open to the sky through rectangular or circular apertures in the roof.<\f>While they appear to be architectural in nature, these spaces exist solely to create the light effects and perceptual events that constitute Turrell's art. To achieve his optical effects, Turrell coordinates a complex system of lights that run in concert with natural cycles of sunrise and sunset, and respond to constantly changing atmospheric conditions." See? Heaven.