By now most people with an interest in art must be aware that a huge and superb Manet retrospective cosponsored by the Louvre and the Metropolitan Museum has just opened at the latter museum here. And that critics are already describing it as an ''eye-opener,'' as affording a ''new insight'' into the nature of Manet's genius.
If that is true, at least as far as the critics are concerned, I cannot help wondering just how carefully they have looked at the many Manets in American collections, or for that matter, the Manets reproduced so beautifully in so many books. There is no question that this is a truly magnificent exhibition, and probably the best Manet show we will see for decades to come. But to stress how revealing it is of Manet's true qualities is to admit to having taken for granted (or to having not really studied) the superb Manets of all periods owned by the Metropolitan, the National Gallery of Art, and various other American museums.
Manet, I'm afraid, is one of those artists to whom extensive lip service but very little real attention is paid. Every-one knows he was a great painter and that he scandalized Paris in 1863 with his painting ''Le dejeuner sur l'herbe,'' a large work that depicted two fully dressed men and two unclothed women picnicking in a park. But he has never quite caught on with the public the way Monet, Renoir, Degas, Van Gogh, or Lautrec has.
But then, that's not surprising. As a painter, Manet is a bit too cool, quiet , and detached, and too indifferent to painterly flair, ever to be truly popular with the public - or even with many art professionals. Art historians certainly give him very high marks, but he actually belongs more to his fellow painters, and to all serious students of what it means to paint, to build form through pigment, or to create harmonies through subtle adjustments of values and hues. He was, first and foremost, a problem-solving painter whose genius and art-historical importance lay in his ability to set and to resolve remarkably relevant and challenging painterly problems. To make anything else out of him, to stress his relevance to 20th-century sensibilities and realities on the basis of his work's ''modern'' look or the directness of its execution, is to put the emphasis on his art's appearance rather than on its substance.