Thomas Cole - The Angel Appearing to the Shepherds.1833-34.
Here's a list of artists who left secrets embedded in their artworks. Leonardo da Vinci is the classic one, secretly painting a brain in the Creation of Adam for example. My take on that is to highlight the mental construct of the universe veiled through Catholic dogma.
These secret codes were either occulted into artworks because they are too juicy to just put forward straight up, or in the alchemical tradition, because the deeper subconscious information can not be portrayed so much in words but more through symbol.
This is the beginnings of a list which I'll keep adding to and adjusting at this post here through time. Sharing as well with those who have eyes to see. Much to discuss regarding.
Leonardo da Vinci
Heinrich Khunrath
Matthäus Merian
Arnold Böcklin
Thomas Cole
Johann Daniel Mylius
Nicolas Flamel
Gustav Dore
Basilius Valentinus
Johann Valentin Andreae