IT HAS always reassured me as I yawn through classes and sleep through lectures to realize that professors aren't just sentenced to their teaching. Many do things on the outside, Marty Peretz has followed a successful career sinking money into losing presidential campaigns. A whole array of professors have proved themselves as "advisors" in Washington and John Finley has the A.D. Club. But little did I expect that Assistant Professor Donald D. Bacon, whose sinisterly analytical lectures scared me away from his American Drama: Studies in the Dramatic Imagination last fall, should direct an entertaining play at a time and in a place where entertainment is such a rarity.

The source of the production's success lies somewhere near the larynx of Christopher Josephs, who plays Henry IV. The play doesn't really exist until Henry enters and when he does it exists only on the terms of his mad role-playing. Josephs plays a one-man show, first delivering lines with a monarch's dead earnestness, then echoing himself and finally ho-ho-hoing with the most amusing laughter even to open at the Loeb. At least when Josephs ho-ho-hoed, I ho-ho-hoed--while the rest of the audience kept its peace.

I don't know who these Loebies are trying to fool calling the Thursday evening performance "opening night". As far as I could tell it was a dress rehersal that meant business. The only people in the audience seemed to be those in some way connected with the production, and most of them must have seen it before because they made no effort to react to what was happening onstage. At these Thursday evening performances it's best to keep your month shut during intermission because the critical comment you make is bound to offend the person who sewed the gold buttons on such-and-such's jacket. But as a courtesy to those new to the production these initiates might try to applaud and laugh it up more convincingly. When you laugh alone the Loeb can be lonely.

ALTHOUGH JOSEPHS steals the show when he enters in the middle of the first act, several other members of the cast do fine jobs. Ellen Olian-Bate as the aging but still passionate Marchioness of Tuscany was probably the most professional. William Fuller delivered his lines too quickly and mechanically at first but later did very well as the ridiculous Doctor Genoni. Henry's councillors Robert Hershman, Brian Powers and John Rudman seemed to capture the foolishness of their roles.

What still puzzles me about the play is that it should be described as a tragicomedy in the program. It seemed like a light comedy, and what wasn't comedy seemed strange rather than tragic. If there was a truly tragic element it must have risen with the dialogue's hot air because the final action, while unexpected, was also unaffecting. If the play was ever a tragicomedy, Bacon must have decided to stress the humorous elements over the serious.

Perhaps Henry IV should be more moving but if so I don't know whom to blame. A play whose theme is madness and the illusion of really should not be as easy to brush aside as this production. When Adams House did Marat-Sade two years ago it was so emotionally convincing that even the audience seemed to be going crazy. But that was two years ago and may be the audience was.