IF OUR EYES AREN'T deceiving us, public broadcasting -- originally the poor little bookworm brother of big-time radio and TV -- is looking more and more like a cathode copy of anything you hear or see on the regular networks. The soaps may be soapless and the movies a little older on your public stations, but these outlets do offer programs with mass appeal; and even though public broadcasters like to tell you that ratings aren't the end-all and be-all, they've been known to take a head count every now and then, too. The big distinction, of course, has been that public broadcasting has been commercial-free. So guess what's coming next to your friendly neighborhood ad-free TV station?

Commercials. If the Senate accepts an amendment approved by the House, 10 public radio stations and 10 TV outlets around the country -- though not Washington's WETA-radio/television on this round -- would be authorized to run advertisements. Each station could carry up to four minutes of ads an hour, at the beginning or end of a program, for local audiences only. Political or religious commercials would be prohibited. The experiment would be for 18 months, with an option to renew and expand after that.

Many broadcasters, while wary of this scheme, are willing to give it a try because they say they need the money. Neither the Reagan administration nor Congress is talking big dollars for public broadcasting anymore, and station managers are eager to add extra green to their screens from almost any source. Still, as Peter M. Fannon, general manager of the National Association of Public Television Stations, says, they do fear that advertiser pressure might alter the special character of their programming.

So do we -- and once it does, what's so all-fired different about "public" broadcasting? Advertising does have its rightful place, as anyone here in our building can tell you; but what's the point of government or individual subsidies for public broadcasting when it, too, goes commercial? Once that distinction is blurred, some adjustment to your set -- flicking it off -- may seem necessary.