YOU could say Rembrandt is not exactly what he used to be."Leaner and fitter" is the phrase used by Christopher Brown, Rembrandt specialist, to describe the current scholarly view of the great 17th-century Dutch artist. This more selective view of what Rembrandt did - and did not - paint is the result of rigorous efforts by art historians in recent years to build a truer picture of him. "Basically gone," says Dr. Brown, are "the more sentimental aspects of Rembrandt, particularly those many studies of the heads of old men.... That kind of picture has fallen away - disattributed in one way or another." Although many "Rembrandts" have been taken from him, there are also some (rather clumsy) paintings of his earliest years that have been given to him - so there are additions as well as deletions. Among the losses are some rather spectacular works including Berlin's "Man with the Golden Helmet," Chicago's "Girl at the Door," and (maybe) New York's "The Polish Rider." It should be said, however, that if you phone the Frick Museum, which owns "The Polish Rider," to ask how they have labeled it now, you are firmly told: "It's still a Rembrandt!" In Britain, says Brown, "there have been a lot of reattributions.... The principal loser in this respect has been the Wallace Collection." (Like the Frick, it is an outstanding personal collection turned public gallery.) The Wallace Collection according to Brown "went into the 20th century with 11 Rembrandts and now, I think, has one." Brown is chief curator of London's National Gallery, and one of the three selectors of works in the major Rembrandt show, which was seen in Berlin and is moving to Amsterdam Dec. 4, and finally to London next March. One of the purposes of this exhibition is to make the case for a number of recent disattributions. The main body of 50 paintings (there is a drawings and etchings side to the show too) are Rembrandts that have never been doubted. But then come works that were, until recently, considered Rembrandts (all of them were listed as such in Horst Gerson's definitive catalog of 1968), but are now thought to be by such lesser-known names as Gerrit Dou, Samuel van Hoogstraten, Ferdinand Bol, Nicolaes Maes, Willem Drost, and others. These new attributions are supported by pictures known certainly to have been done by the artists in question - artists who were mostly shadowy figures, but whose characters and repute are decidedly enhanced by their being reconsidered as the painters of works loved for many years as Rembrandt's. Has Rembrandt's stature been diminished by such scholarship? Brown thinks not. The sifting, he says, makes it "clearer what his achievement is.... It strips away ... Rembrandtesque pictures that have been attributed to him subsequently." He says that although some individual pictures may remain "in dispute," there "is no doubt that the general tendency to take a number of pictures out is the right one." Much of this revised assessment of the works of Rembrandt (1606-1669) and consequently of his pupils, assistants, and followers, has been pushed methodically forward by the Holland-based Rembrandt Research Project. This project has been working away chronologically through each year of the painter's career for approximately the last quarter century. But its last published findings reach only up to 1642. So any Rembrandts datable later than 1642 that have been disattributed have been removed from the oeuv re by the consensus of other scholars or institutions. "The Man with the Golden Helmet" for example, would come, if it were a Rembrandt, from the 1650s. The Berlin Museum was responsible for taking this work, virtually a national icon in Germany, away from Rembrandt. It caused quite a stir. The Polish Rider, says Brown, "is one of the most famous Rembrandts - in this case in America, and much admired. It was a bold throw to deattribute it and give [it] to Willem Drost." This was done in a review in a Dutch journal written independently of the research project by one of its members. The case of "The Polish Rider" (it is not shown in the exhibition because Frick Museum works cannot be loaned) seems to Brown particularly significant of the way in which perceptions of a work can be significantly altered not just in the popular imagination, but in the thinking of even a practiced authority like himself. He doesn't agree with a number of the disattributions made by the research project, and he is not sure he agrees that "The Polish Rider" is not by Rembrandt either. ve always admired that picture very much indeed. I think it's very, very beautiful. In a way it was always one of my touchstones of what Rembrandt was. But then once this shadow of doubt passes over it, you go and look at it and think 'Well, it is very untypical for Rembrandt 'Did he really work like that? - would he have painted a horse like that, is the landscape like his other landscapes?' I mean it does make you question pictures, makes you think again about them." That a shadow of doubt should trouble an expert in this way matters because of a movement in Rembrandt scholarship back toward good old-fashioned connoisseurship - to the importance of the eye of Rembrandt scholars, pitting their expertise, their educated and intuitive recognition of an artist's touch, against each other to arrive finally at a body of agreed opinions. This respect for mere human ability in an age of technological wizardry - of paint samples, of X-rays to study underdrawings, of reflectog rams, of infrared, of dendronology to date wooden panels, even of thread counts to compare or relate canvases - may seem apparently regressive. But the successive prefaces to the various reports of the Research Project chart this change of approach. The project, Brown says, was "set up in the '60s when these means of technical analysis were just being brought to bear on paintings.... And it must have seemed at that time that many of these problems of attribution could be solved by technical means - that this really was a very significant step forward." The prefaces of the Rembrandt project publications said as much. But it has been found that technical means are limited. "I mean you can identify fakes, and later copies, and anything that is quite different from a technical point of view. But if you are actually telling the difference between a painting by Rembrandt and by Ferdinand Bol [who worked in Rembrandt's workshop] it's not going to help you very much because, of course, by the nature of the apprentice system in the Netherlands in the 17th centu ry, the pupil learnt how to work like his master - he built up his paint in just the same way. So for that you have to go back to traditional connoisseurship. And that's what they admit they've done in the preface...." Then - the cynic might say - we are back to the table-tennis games played by the academic specialists, where opinions and arguments are hit back and forth until the hardest and straightest shot wins. Brown finds some truth in that, "but on the other hand I think it's not an entirely futile exercise. There can be no doubt that today we know more about the historical Rembrandt, what he actually painted - though it may be further refined - than we did 20, 50, 100 years ago. So there is a sense in which we a re getting closer to the historical figure of Rembrandt." That this is now again being achieved by specialists "who simply rely on the evidence of their own eyes" is perhaps rather encouraging. It's nice to know that human beings still have their uses.