The biggest mystery on television these days isn't to be found in the gumshoe series, but in the ever-enthralling realm of commercial advertisement. It might be entitled: "The Case of the Vicarious Husband." But more anon.

The last thing the people who concoct TV ads want to do is to make waves. On the other hand, they spend their working lives rebounding from the waves made by others, including their competitors. It follows that styles and trends in TV commercials tend to run in cycles. When the "hot" idea of the monent is scientific-looking analysis, for instance, then every other ad is apt to feature a bespectacled septuagenarian in a white coat explaining how the extra added ingredient in his pain-reliever works overtime to deliver instant surcease.

There was a long while, more or less coincident -- and not accidentally, either -- with the Watergate era, when the on-location, cinema-verite testimonial descended upon the tube like a plauge. Mike in hand, the pitchman would corner his "unsolicited" prey in, say, a laundromat, where she (these subjects being almost always female for all too obvious reasons) could be prodded into gushing endorsements of cold power or the newest bleach or whatever else was going down at the time. Such recitations were marked by awkward pauses, ungrammatical constructions, colloquialisms and faulty sentence structure intended to assure us of the realism of the scene, despite the patently staged look. Watching seven or eight of these in a row was sufficient to give one a terminal case of ring-around-the-choler. But like everything else on television, the fad passed and the new ones took its place.

Remember Mrs. Marsh, the Colgate babysitter, the skinny lady with the teenth (one had visions of Central Casting sending out a call for a woman of homey, ingratiating manner and a set of incisors of maximum size and visibility) who lectured the kiddies on the importance of frequent and regular scrubbing? Why, not long ago she was so omnipresent on our screens she seemed practically a member of the family. Her zeal was disarming, but one suspects her emaciated appearance may have been due to excessive dental prophylaxis, say, 47 times every hour. In any case, we miss her pearly presence, for she seems now to have retreated into the mists of television Nirvana -- maybe she brushed herself out of existence.

Of late, some sponsors, perhaps in hope of tapping the still rampant nostalgia craze and its spin-off, the reliving-one's-youth syndrome, seem to be recycling their own cycles. There is, to stick with dentifrices for the moment, the Crest series, for example, in which we are greeted in home surroundings by a personable young woman, who says something like the following:

"Hi, I'm Janet Flosspuller -- this is me [holding up a photograph of herself at an earlier age] when I was one of the Crest children in our school experiment. I'm grown up now, but I'm still getting great checkups."

We note sadly, however, that the poor kid has a slight speech defect suggesting possible dental involvement, and note further, with no small puzzlement, that nothing whatever is said about the present condition of her teeth, which for all we know may be rotting out of her head. The only thing we're told is that her "checkups" are great, which would seem to be a different thing altogether. Or does Crest mean for us to be brushing our checkups twice a day?

As to current trends, what's come to the fore recently is an overbearingly paternalistic approach in which the sponsor is presented as a benevolent, all-seeing, all-knowing father figure, who will lead us benighted souls to salvation -- these ads often have an air of confessional or religious conversion about them. The unspoken creed behind them is: What's good for the firm of Pockmark and Grumble is even better for The American Family.

One such goes more or less like this: An offscreen voice says "We're fooling Mrs. Conworthy..." (The "we" is ambiguous -- you see, you and me out there in viewerland are brought in as accomplices to this conspiracy; it makes one feel practically like a stockholder.) "... We've told her we've taken the whitener out of Final Touch."

But the ad that illustrates the trend most poignantly -- and the one that brings us finally to our mystery -- is the White Cloud series. Of course, one can understand the predicament of the advertisers when it comes to the topic of the downiness of toilet tissue. One can see the Madison Avenue brain trust trying desperately to come up with ways to put the matter with a proper degree of delicacy and tact, and yet still get the message across. After all the welfare of the Great American Sitzfleisch is at stake.

Charmin solved the problem charmingly (no pun intended) by intimating that their product is so irresistibly pliant that even the hardshelled store manager Mr. Whipple (of course, we know him for a softie at a glance) cannot prevent himself from cuddling and fondling the wrapped rolls of tissue right off the shelf.

The current White Cloud commercials, however, are composed in the manner of a catechism or inquisition. A woman is seated at a table. Facing her, his back to us, is Big Brother announcer. He asks her if her husband cares about softness in bathroom tissue -- we can smell a setup a mile away. She replies, offhandedly. no, he doesn't care. Well, let's see, says Mr. Smartypants, and at this point he indicates a TV set at the end of the table. He clicks a switch, and a masculine figure appears on the screen, also sitting at a table, upon which are two rolls of guess what. "Who's that?" he demands. The woman looks at the screen. "That's my husband, tee-hee, tee-hee, tee-hee." (These admissions are always accompanied by inexplicable bouts of uncontrollable giggling.)

The husband is next asked which of the two brands in front of him he preferred; and when he tells us White Cloud, he is further importuned for his reasons. "Because it's softer." Now the inquisitor turns and pounces on the defenseless spouse, closing the insidious trap. "You told us you're husband doesn't care about softness," he says, in the patronizing tone one uses with infants caught in a fib. The epiphany is at hand, the conversion complete. Under his further goading she confesses her error, and promises henchforth to purchase only the Right and True product. And why? "Because I want to please my husband."

It's probably unnecessary to bore the reader with the details of a study of husbands who have used brands other than White Cloud over the years, but who have, quite unaccountably, been pleased by their wives anybow, possibly by different sorts of ministrations -- who knows what wiles women may have.

We come now, however, to our strange purrle: Why is the husband seen on a TV set? Why is he not there in the flesh, at the same table as his wife, to present his testimony?

There is the possibility, of course, as some of you may already have surmised, that this man is not the real husband at all, that the real husband may be bound and gagged in some closet somewhere vainly trying to scream out his preference for another brand, while a cleverly disguised actor, abetted by video distortions, passes himself off as the genuine article.

But the actual reason is more likely that the sponsor is smarter than the rest of us after all, in having realized that for a public reared on television compared to TV, real life isn't convincing enough -- it doesn't sell. If we don't see it on TV, we don't "buy it."

It's one of the most bewildering paradoxes of contemporary society that television, possibly the medium most easily subject to distortion, manipulation and misrepresentation of reality, has become for many of us the one absolute test of authenticity, of what's real. We used to say, "I read it in the papers," and that sufficed to prove that something had actually happened, but "I saw it on TV" has a thousand times the force, because seeing is believing. Add to that the authoriative tone of TV announcers and admend seen or unseen, and the vast reach of electronic broadcasting, and you've got the possibility of conning scores of millions at the speed of light.

The husband in this commercial is seem on TV because that's the way to certify his credibility, for the wife, and doubly, for us, the audience. She sees him on television: we see him on a TV screen within a TV screen -- it's television squared, twice as convincing Maybe the next step will be an infinite regress of TV screens, at the apex of which we'll see -- ourselves, surrounded by all the choice wares of the marketplace.

Onscreen, we see the aghast face of the homemaker who exclaims, "Put it baghck!" (The word is "back," but it's pronounced in the accents of some regional argot impossible to render orthographically.)

"If my little boy goes around in gray, dingy undershirts," she continues, "how's that going to look? What kind of mother am I?"

This question is not answered in the course of the commercial, thus prompting some research on the subject. It has been established independently that Pat Boone, Mick Jagger and Robert Redford ran around in notoriously gray, dingy undershirts, and that, by way of contrast. Charles Manson's undershirts were sparkling white. Yet, curiously, in the opinion of objective experts, these facts seem to have had relatively little bearing on the competency or efficiency of the respective mothers. This would seem to suggest that factors other than whiteness of undershirt may be involved here.

In any case, all is set right. Our unseen patriarch assures the despairing woman that it was all in jest, that they wouldn't dream of removing the whitener because that's what makes the product so special.

Now we see a face of beaming relief, as she declares, "I'm pleased. I'm tickled to death." CAPTION: Illustration, no caption