It's hard to imagine that Kate Middleton can walk among the commoners unnoticed these days -- but that's exactly what the Duchess did when she visited the National Portrait Gallery in London Wednesday.
The 29-year-old spent the afternoon touring the contemporary collection downstairs with a small group of curators, an onlooker tells Us Weekly exclusively.
"She was dressed all in black -- she must have been boiling because it was a really hot day in London -- in knee-length black boots, black tights and a slim-fitting black dress," the onlooker says. "She had her hair up, completely pulled up into a twist at the back."
Middleton, who studied art history at St. Andrews (where she met future hubby Prince William, 29), wasn't fazed by some of the more controversial pieces, including Marc Quinn's Self, a self-portrait cast of the artist's head made from his own blood and then frozen.
Because the gallery downstairs is split into many side rooms, Middleton was able to move around without too many people noticing her -- though one tourist did attempt to snap a photo of the Duchess. "Her security guard suddenly appeared and told the man he should delete the photo, but other than that she wasn't bothered at all."
"She didn't spend a huge amount of time looking at it," the onlooker notes. "She just smiled and admired it briefly."