An Early Christian Sect, the Neoplatonists, believed that the universe was a musically harmonious system, symbolized and made visible by light, which came directly from God. Architecturally applied, this concept gave their cathedrals an ethereal quality; the walls, instead of sealing off interior space, appeared to link it with infinity. A welcome throwback to Neoplatonism is this Long Island beach house designed and built by architect Earl Combs for Steve Ostrow, the 39-year-old proprietor of a chain of luxury health spas (one of which, the Continental--nicknamed The Tubs--has lately become one of Manhattan's most "in" night clubs). Don't be misled by the somewhat cloistered entrance (above); that's the boardwalk side. The other facade, facing the sea, is so open as to create a seemingly symbiotic relationship between the dwelling and its setting. That side of the house is almost entirely glass, in the form of tall panels and large sliding doors that enable Ostrow to open his place to the elements, and it embraces an outdoor dining area shaded by a sun screen. During the day, light streams in that side of the house, and through slits in the bedroom walls and portholes in the bathrooms and kitchen. The sea, reflected by mirrored doors in the kitchen, bedrooms and bathrooms, is ever-present. And, just as the sea provides a sense of unlimited dimension, so does the interior of the house--which, like Oriental boxes carved one within the other, is an ingenious exercise in spatial economics. Reflectors, such as mirrors placed between study and dining room, subdivide and circulate the space. Each area seems to flow into the next, and the guest's eye is never trapped. When the bedroom blinds upstairs are open, you can see clear across the house, including the skylighted, coffered ceiling that overhangs the two-story living room, which is reflected below in the geometrics of the dining table, coffee table and built-in furniture. The general feeling of weightlessness is countered by the 14 square columns, which, in addition to providing support, house such essentials as wiring, plumbing and a four-speaker hi-fi system--thereby combining structural, aesthetic and utilitarian roles. Completed in August 1970 at a cost of $60,000, the house admirably fulfills the objectives of architect Combs: "On a fine site directly facing the beach and the ocean, to create an enclosed space that would look to and reflect the views of the beach and sea; to handle the very strong light and glare without losing the view and a sense of openness; to provide a plan that would balance and organize the interior space, while maintaining visual privacy and sound separation when desired; to utilize low-maintenance materials." A rather tall order; but the musicality of Combs's ingenious design transformed a relatively confined area into a piece of architectural wizardry--and one hell of a place to spend a weekend.