* Experimental films have also been unspooling at the Museum of Modern Art in New York, where a fascinating Joseph Cornell Exhibition has been supplemented by surreal cinematic collages assembled or commissioned by Mr. Cornell, with the assistance of such artists as Larry Jordan and Stan Brakhage. And another key experimentalist, Stan VanDerBeek, recently visited the museum, bringing some of his latest works in progress. The most exciting was his "steam screen" setup, in which a dreamy three-D effect is created by projecting abstract shapes on a shimmering curtain of steam. Less impressive are his attempts to reproduce dream-states with conventional movie materials. But most of these experiments are still in flux, and Mr. VanDerBeek's skill as an animator and image manipulator -- his "Breathdeath" is a genuine classic -- make one curious to see where his latest excursions will end up.