Walking into the Marc Jacobs show tonight, a retailer remarked, "We have to come here in order to find out what we'll be seeing next season." His point: The lush colors and Lurex that have been everywhere this week were inspired by the ode to the 1970's Jacobs delivered last time out. Well, come next fall, you'd best prepare yourself for plenty of polka dots, a good deal of latex and lace, and a much more fitted silhouette. Backstage, Jacobs himself said the new collection was a reaction against the loose, fluid feeling of his Spring outing. "I wanted something strict and severe," he said.