LIFE IS SHORT, and the art of Jasper Johns is long. So long that one may long for the end of the exhibition of Johns drawings at the National Gallery of Art.

It is surprising, even dismaying, to learn that this show of 117 drawings comprises only about a quarter of those Johns has sold so far during his 35-year career, for nothing seems to have been left out.

There are lots and lots of examples of the many periods of this "leading postwar American artist," such as his flag period, his map period, his target period, his crosshatch period and -- as it is not possible not to resist calling his bathroom series -- Johns's john period.

The works are beautifully presented, and a handsome brochure attempts to explain Johns's murky-quirky evocations of "things the mind already knows." But then the interpreter throws in the towel:

"Johns' recognizable images, as well as his methods of juxtaposing them, his choice of materials, his many ways of making marks, and his handling of color, all function as signs. These signs propose a range of possible meanings for each work. It is up to individual viewers, who bring different histories and psychology to this art, to decode the signs and, led by Johns' clues, to imagine connections among them. The viewer thus assists in creating meanings for the works, which invite multiple interpretations."

This do-it-yourself injunction is a revealing commentary on much of modern art. What is the artist's role -- his excuse for demanding our attention and support -- if not to share his special vision and make his own points?

One superb drawing/collage shows that Johns indeed does know how to make a powerful and straightforward statement: "The Critic Sees" (1962) depicts a pair of spectacles with mouths where the eyes ought to be.