Christmas tree lighting dates to 17th century Germany, when wax was melted to tree branches to hold candles to illuminate specific ornaments. In 1882, Edward Johnson, an assistant to Thomas Edison, designed the first set of electric tree lights, which became popular in department store displays. It wasn't until the introduction of safety light strings in 1917, however, that household tree lighting took off, spawning innumerable combinations of colors, shapes, sizes, and figural lights.