In the early 1800s, navigation was basic. Although ship captains did have some tools to guide them, they still relied heavily on eyesight. This was especially true when they were close to shore. Captains were on constant lookout to make sure their ship didn't run aground. Lighthouses were constructed to make the shore easier to see. The problem was that lighthouses had weak lights. They could not be seen from far away. This problem was solved by a French physicist named Augustin-Jean Fresnel. Fresnel invented a lens that would focus light into a single source. This focused light could be seen from many miles away.