Built in 1861, this is one of the twin lighthouses that stand on Thacher Island off the Rockport coast. Today, the north tower shown here is owned by the U.S. Fish and Wildlife Service and maintained by the town of Rockport as part of a National Wildlife Refuge. The south tower is owned by the town. Compare this view of the north tower with this stylized view showing it at night. For the account of the shipwreck that gave the island its name, go here.
This postcard was published by the Rockport Photo Bureau. There is no date or postmark. Based on the similarity of the markings on the reverse to other postcards, I estimate its date to be around 1920.