What is a devils eye on old house for?

A devil's eye is an apotropaic symbol or talisman designed to ward off evil. They are commonly employed throughout Asia and are often painted on the prows of fishing vessels. The eye appears to be staring outward, watching for evil spirits. These symbols are most powerful in the shade of blue. Sometimes called "nazar", which means amulet in Arabic, they are popular in various countries of the Mediterranean, Western Asia and South Asia.