William Etty - Candaules, King of Lydia, Shews his Wife by Stealth to Gyges, One of his Ministers, as She Goes to Bed (1830) NSFW

I'd never heard of this, and couldn't figure out what was going on. Wikipedia says:

It shows a scene from the Histories by Herodotus, in which Candaules, king of Lydia, invites his bodyguard Gyges to hide in the couple's bedroom and watch his wife Nyssia undress, to prove to him her beauty. Nyssia notices Gyges spying and challenges him to either accept his own execution or to kill Candaules as a punishment. Gyges chooses to kill Candaules and take his place as king. The painting shows the moment at which Nyssia, still unaware that she is being watched by anyone other than her husband, removes the last of her clothes.

Etty hoped that his audience would take from the painting the moral lesson that women are not chattels and that men infringing on their rights should justly be punished, but he made little effort to explain this to audiences. The painting was immediately controversial and perceived as a cynical combination of a pornographic image and a violent and unpleasant narrative, and it was condemned as an immoral piece of the type one would expect from a foreign, not a British, artist.

which is great.

The Tate have it, but it's not up.