BY NOW JUST ABOUT everyone has rejected everyone else's explanation of Jimmy Carter's new hairdo. Jody Powell has given the nation to believe that the changeover is without overriding political, social, economic, technological or other meaning. Gerald Rafshoon has alleged that, God knows, it has nothing to do with image-making. And the president himself has fogged up the speculation about the liberal versus conservative implications of moving his hair part from right to left with the reassurance that he has done so "only for the primaries." For the November election, he pledges, it will go "right down the middle."

Well and good. Except for the telling photographs we reproduce below. What is the meaning of the Great Emancipator's 1864 (on the right) to 1865 (on the left) switch? Actually, these photos have only reinforced our suspicion that the real explanation for all this is to be found not on Madison Avenue nor in the recesses of the Carter administration's 1980 game plan, nor even in some barber's distracted inadvertence. No, as is so often the case (and surely the First Sunday School Teacher must himself have suspected as much), the true explanation is much more spiritual than that. May we remind you of what the holy scriptures said:

"Vanity of vanities, saith the Preacher . . . all is vanity . . . One generation passeth away, and another generation cometh . . . The thing that hath been, it is that which shall be; and that which is done is that which shall be done: and there is no new thing under the sun." CAPTION: Pictures 1 and 2, From "Lincoln in Photographs" by Charles Hamilton and Lloyd Ostendorf, Copyright (c) 1963, The University of Oklahoma Press.