At Riverhead Long Island splits into two narrow strips of land, each about forty miles long and each with a distinct character and topography. The South Fork stretches east into the Atlantic Ocean from the Shinnecock Canal to the majestic bluffs at Montauk Point. Dotting the coastline are the stylish Hamptons--Southampton, East Hampton, Westhampton Beach, and Bridgehampton--and villages of Sag Harbor, Amagansett, Watermill, and Sagaponack. In contrast Long Island's North Fork is a pastoral quilt of vineyards and farms by the sea, a place of serenity and beauty.   Renowned photographer Jake Rajs has captured the spirit of both the North and South Forks--the rocky coastline, the calm waters, and the vast expanses of fields and wetlands of the North Fork and the range of the South Fork--juxtaposing formal privet hedges and pumpkin fields, crashing surf and serene coves, fishermen and polo players, contemporary houses and modest shingled cottages.   Most important Rajs has captured the light throughout the day, from misty dawn to the vivid colors of sunset.