A FREE guide to "Circa 1492" translated into six languages is what National Gallery director J. Carter Brown calls "an indispensable Linus blanket to take around with you."The guide is just one of several examples of how the gallery wants to make its "Circa" show user-friendly. He explains, "The educational dimension is probably more serious in this one than in anything we've ever done because this is, as you will see, a thinking person's show." In addition to the usual acoustical guide, there is a supplemental approach so that the audio and visual can work in tandem with each other. For families with children, there is a special recorded tour that points up what might app eal to the young but will help inform those of any age. He says the gallery is going one step further with a film that takes one aspect of the show, the application of the new scientific and mathematical discoveries on perspective, and brings it right into this century in a special effects studio in Hollywood. It is called "Masters of Illusion," made by award-winning director Rick Harper. Mr. Harper said that one of his sons has a birthday coming up, and "he's been bugging me for the little fantasy action figures in the film whose names happen to be Leonardo, Raphael, Donatello, and Michelangelo." With 30 minutes for the film, Harper says, "We decided to zoom in on one area, the work of the masters of the Renaissance and in particular the development of illusionary techniques for establishing the effects of 3D on a two-dimensional surface. "When you think of a Raphael who was looking at a large flat wall that he had to put an epic scene on, he was dealing with a lot of techniques that they developed at that time, of 3D on the staging, lighting, and color.... We even talk about Andrea Mantegna and his selection of camera angles...."