Lamech Kills Cain
Circa 1590-94
Palazzo Corsi-Tornabuoni
A well-known interpretation of Genesis 4:23 had it that Lamech was blind in his old age but went hunting nevertheless. He had a boy help him aim, but even with that assistance he accidentally killed his ancestor Cain, who was lying in a bush.
Cain was Lamech's great-great-great grandfather, so the artist gives him a very long white beard.
Read more about images of Cain's story.
Source: this page at Wikimedia Commons.