Family mystery with a literate touch but declining interest;

NEW YORK Why did Wendell Bradford, a future potential Pulitzer Prize poet, leave home at 16 and then disappear from public view? Confident Harvard man Victor Salt (John Vickery) probes the question and comes up with conflicting answers in Percy Granger's ''Eminent Domain,'' at the Circle in the Square Theater. The inquiry takes place in 1975 at the home of Wendell's irreconciled parents in a Midwestern university town seven years after the son's departure. Holmes Bradford (Philip Bosco), whose possible appointment to Brandeis would rescue him for these scrubby groves of academe, welcomes the visiting scholar and appears disposed to help him. On the other hand, Katie Bradford (Betty Miller), a recovered drug addict and alcoholic, refuses all cooperation. (''Eminent Domain'' contains reminders of O'Neill's ''Long Day's Journey Into Night'' and Albee's ''Who's Afraid of Virginia Woolf?'') The ''Eminent Domain'' of the title is what Salt regards as the public's right to learn family facts and history that will help it to understand Wendell Bradford and, incidentally, help the sales of Salt's book. The politely intrusive visitor is an academic who will not perish for want of publishing. Mr. Vickery plays him with a nice balance of courteous deference and discreet opportunism. The subjects of these Salt talks are also carefully observed and believably portrayed. The ever resourceful Bosco presents the image of a shaggy-haired, tippling academic -- sharp-spoken yet impulsively good hearted, a Harvard graduate who fumbled the main chance and has spent the remainder of his career amid the alien corn. As the withdrawn Katie, a gifted painter who abandoned her career, Miss Miller reveals the subtleties of difference between strength of character and uncompromising stubbornness. Along with the principal exchanges, Mr. Granger has written a couple of neatly turned incidental scenes: One involves an embarrassed freshman (Scott Burkholder) who must con Bradford as part of a fraternity initiation. The other concerns a British professor (Paul Collins) whose revenge over being denied tenure provokes a campus protest and sparks Bradford's slightly scandalous reaction. The problem with ''Eminent Domain'' as theatrical entertainment seems twofold. Its prolonged series of confrontations and revelations tend to decline in interest as the action unfolds, leading to a conclusion more contrived than resolved. The play nevertheless displays a genuine literate touch plus a concern for both intellectual and human values. The admirable Circle in the Square production was staged by Paul Austin, with scenery by Michael Miller, costumes by Jennifer Von Mayrhauser, and lighting by Lowell Achziger.