"The Daughters of Edward Darley Boit" by John Singer Sargent
When he completed this oil painting Sargent titled it "Portraits of Children" probably to put on display his awesome portrait painting skills, in order to gain more commissions. It certainly is a portrait of children, but a conventional portrait it is not. A conventional portrait depicts the sitter or sitters as the primary subject, with the background or "environment" subordinate to the subjects.
Sargent hasn't created a picture of children, he has created an environment with children in it. Big difference. And I believe, the first reason that this painting is so captivating.
The space behind the elder daughters that is framed by those two vases along with the hints of light reflection in the room beyond, creates a sense of mystery that makes our eyes wander back there, holding our attention all that much longer.
Even more interesting than that alluring space however, is the different ways in which Sargent has depicted each girl. I was always amazed by how random the poses seem and yet how they work together to draw the viewer into the picture plane. My eye goes from the youngest in front, to the older two in back, who, like the space they pose in front of, is rendered very loosely yet convincingly. But that looseness of the older two contrasts nicely with the more exacting rendition (and seemingly more formal poses) of the younger two in front, and just ads to the mystery of the overall piece.