Laura Cumming

Some art historians have spent forever trying to work out if this is an upper room because you can only see the top of what might be a tree or a lower room because it's a bush.

We can't really tell.

Right at the very back of the painting on the wall...

is the most clinching detail, if you can call it a detail, because in fact it's the advent of a whole new way of painting.

It is a convex mirror.

Anyone listening to this can think of convex mirrors of the sort you see in your aunt's house, or you might see in a junk shop somewhere, and reflected in that mirror are two little figures.

We'll come back to them.

And above them, finally, is this immense...

beautiful, very complex, very, very expensive chandelier with one candle a light.

We'll come back to that in a moment.