This is one of many illustrations from "The Forsaken Merman", a rhymed lyric poem written in irregular metre by Matthew Arnold, begun while he was in college in the early 1840s -- about the same time Cyanotype was invented.
 
The basic premise recurs in Danish, Norwegian, German, and Slavonic folklore. The Merman, a King of the Sea, marries an earthly maiden and lives with her happily for many years, but at last she leaves him for a visit to her friends, promising to return. Time passes, but she comes not back. Scruples of conscience have arisen, and she chooses, as she thinks, between her soul and her family. The story is told by the old Sea King to his children.