THIRTY YEARS AGO, when the Supreme Court first spoke about secret criminal trials, it could find no record of one being held in an American or British court since 1641. The court will have no such trouble this year when it speaks on the subject for the third time in its history. The number of secret trials is increasing almost daily and seems almost certain to keep on growing unless the court turns abruptly away from the sympathetic view of secrecy it expressed last summer. Even some of those who profess to be friends of public trials are deciding that a little secrecy is sometimes a good thing.

In Virginia, for example, Attorney General Marshall Coleman has proposed legislation that says "all criminal proceedings" are to be open to the public. But there is a clause that precedes that noble statement of principle: "Except as provided in this act. . . . " Then come the exceptions.

To be fair about it, Mr. Coleman has listed among the exceptions some exclusions that do not impinge on the public nature of trials. Judges have long had the power to bar minors from trials at which obscene or salacious testimony is expected or to limit the size of the audience or to keep specific individuals out of a courtroom because their presence in itself may intimidate a witness or threaten the fairness of the trial.

But his other exceptions are a horse of another color. The major one would permit secrecy in criminal proceedings, other than those conducted in the presence of a jury, in order to prevent prejudicial publicity. While this may well be in line with the thoughts of a majority of the Supreme Court these days, it opens the way for secret indictments, secret preliminary hearings, secret bond hearings and a host of other secret judicial actions about which the public could learn only secondhand.

Still another exception would permit a court and jury to hear in secret the testimony of an undercover agent or informer -- if letting the public listen might "substantially interfere with pending investigations or is likely to endanger the agent or informer." The logic of this proposal, much favored by prosecutors, is difficult to follow. Since a defendant cannot be excluded from hearing such testimony (at least, not yet), the protection from reprisals afforded an informant by secrecy cannot be great and may well be deceptive.

It is quite possible that what Mr. Coleman proposes will look liberal after the Supreme Court gets done with the case now before it. In that case, incidentally, Mr. Coleman will be defending his state's right to conduct a totally secret trial. If so, the nation will have retreated substantially from the promise of the Bill of Rights that criminal trials will be conducted in "public."