The painting on the wall seems so incongruous, less in style, but the attitude of Cupid staring right at us, bow down, treading on something, a mask at his feet and his arm as if to draw the curtain between us and the scene.

Is the girl perhaps reading a letter which will reveal to her that her beloved is not as she believed?

Vermeer's scenes are always so exquisite and tense in their quietude, but this one is particularly intriguing & the way her face is reflected in the float-glass of the pane, and the dust on the window is devastatingly good.

Kind of feel Cupid there is somehow a portrait of the artist, and also indicting the viewer.

Going to have to go and read up on this painting now.

EDIT: Turns out the painting on the wall was only discovered by x-ray in 1979, confirmed by extensive analysis in 2017 to have been covered over with paint after Vermeer's death, was restored to reveal it as he intended in 2021. Explains why I felt so baffled to have not come across it before!

https://en.wikipedia.org/wiki/Girl_Reading_a_Letter_at_an_Open_Window

FURTHER EDIT: better article & correction of some details in previous edit. http://www.essentialvermeer.com/cat_about/open.html