Leading lights always come in pairs and alllow navigators to establish a safe line of travel in confined waters by aligning upper and lower lights like notch and bead on a rifle. In order to be used as day marks they are most often similar in design but in varying colours. The lighthouse in the background belongs to the next line of leading lights guiding ships downriver (Compare: http://www.pbase.com/soenke/image/165737176 )