From dusk to dawn for 96 years, an octagonal Fresnel lens beamed from atop the Sandy Point Shoals lighthouse here. For the nocturnal sailors of the Chesapeake, this lens was an essential navigating tool. To antique collectors, it was a priceless jewel that belonged in a museum.

But one day this summer, a vandal caught a glimpse of the irreplaceable French lantern and decided it was simply another breakable object. He turned it into a thousand useless shards of glass with several violent swings of a baseball bat.

There are 20 lighthouses still beaming over the Chesapeake Bay from the Virginia Capes to Havre de Grace and in recent summers the vandals and thieves have gotten to almost all of them. Carson M. Hooper, the U.S. Coast Guard official in charge of navigational aids on the bay, said the problem has grown "substantially worse" year by year.

This summer alone, according to the Coast Guard, there have been 60 acts of vandalism and theft at the lighthouses, buoys, day-beacons and light towers. The replacement value for the destroyed or stolen items totals nearly $50,000. And there is no replacement price for the Fresnel lens. They do not make crystal lenses anymore. The new ones are made of acrylic.

A baseball bat is not the usual tool of destruction. Rather, the lighthouses are more often defaced at a distance by prankish youths armed with BB guns or by older hunters with shotguns. For the aimless hunter, a buoy or lighthouse is just another target, like a tin can or mailbox.

This year, three lighthouses have been the main targets of vandals - Sandy Point, Turkey Point in the upper bay near Aberdeen, and Fort Washington on the Potomac River. At Turkey Point, gunshot blasts have twice smashed the light's acrylic lens, its batteries have been broken and lead wires stolen, according to Dale Dakin, a Coast Guard machinist. In each case, the light was out for a day, endangering the passing ships.

Of the 20 lighthouses, only three have full-time keepers. Ella Mae Keagle, 74, a third-generation lighthouse keeper from Annapolis, says that there would be less vandalism if all the lighthouses were manned. "Nothing like this would happen if there had been a keeper," she said.

Coast Guard officials agree, but argue that advanced navigational technology has, in the words of Hooper, "made lighthouses not as necessary as they used to be." He said the Coast Guard is gradually phasing out the last three manned lighthouses on the bay. CAPTION: Picture 1, Through the haze of a summer heat, the Sandy Point Light continues to shine, but with a temporary light. Photos by James M. Thresher - The Washington Post; Picture 2, Dale Dakin, of the Coast Guard, examines damage.