The skull in The Ambassadors
Hans Holbein the Younger painted two wealthy young men in 1533, surrounded them with the finest instruments money could buy, and then dragged a human skull across the floor of the picture at an angle nobody could read. It is the most famous anamorphosis ever painted and it is hiding in plain sight in London.
What you are looking at
The painting hangs in the National Gallery in London. On the left is Jean de Dinteville, the French ambassador to the court of Henry VIII. On the right is Georges de Selve, a bishop and diplomat, and Dinteville's friend. They are twenty nine and twenty five. They lean on a two tier stand loaded with objects.
The objects are not decoration. The upper shelf carries instruments for reading the heavens: a celestial globe, sundials, a torquetum. The lower shelf is the earth: a terrestrial globe, a book of arithmetic for merchants, a lute, a case of flutes, a Lutheran hymn book. It is a portrait of two men who have the whole measurable world at their fingertips.
And across the bottom of the picture, at an angle to everything else, floats a pale grey shape like a cuttlefish bone or a smear of thumbprint. It obeys none of the painting's rules. It has its own perspective, its own light, its own vanishing direction. From the front it is unreadable.
Move, and it is a skull
Walk to the right of the canvas and crouch a little, so your eye comes close to the surface and looks along it, and the smear compresses into a perfectly drawn human skull, correctly shaded, sitting in the same space as the two men. At that moment something else happens: the rest of the painting falls apart. The men stretch, the shelves distort, the globes go oval. You cannot have both pictures at once. The painting makes you choose.
That is the argument, and it is made with geometry rather than words. Everything on the shelves is an instrument for measuring the world with confidence. The skull is what all of that confidence is worth. It is a memento mori, a reminder of death, and Holbein has arranged it so that you cannot see your own mastery and your own mortality from the same position.
Nobody is sure where you were meant to stand
This is the part usually left out. We know the skull resolves from the right hand side at a steep angle and close range. We do not know what viewing situation Holbein designed it for, and art historians have argued about it for a century.
One reading is that the painting hung in a stairwell or beside a door in Dinteville's chateau at Polisy, so that anyone leaving the room would catch the skull out of the corner of their eye as they passed, receiving the reminder on their way out. Another is that a viewer was meant to deliberately approach the canvas and stoop, performing the discovery as a small ritual. A third points out that a cylindrical mirror can also resolve it, and that such devices were fashionable.
What is not in doubt is the intent. You cannot paint an accident like this. Constructing a correctly shaded skull that only assembles from one oblique position takes deliberate geometric setup, and Holbein did it inside an otherwise scrupulously naturalistic painting.
The other things that are wrong
Once you know the skull is there, the rest of the painting starts to give up details. One of the lute's strings is broken, which in the visual language of the period reads as discord, and this is 1533, the year Henry VIII broke with Rome. The hymn book is open at a Lutheran translation. The sundials, when read carefully, do not all agree with each other. In the top left corner, half hidden by the green curtain, there is a small silver crucifix that most visitors never notice.
A painting that appears to be a display of worldly success turns out to be about a world coming apart, and the anamorphosis is the mechanism that tells you so. It teaches you, in one movement of your body, that where you stand determines what is true.
Why it still works
Five centuries on, the trick has lost none of its force, because it does not rely on taste or on knowing anything about the sixteenth century. It relies on geometry, and geometry has not changed. Any picture built for a single viewpoint will do this to you.
That is the effect our daily puzzle is built on, with the difficulty inverted: instead of being told to move to the right, you have to find the one direction yourself. If you want the plain version of how such a thing is constructed, read how pavement art works, or play today's puzzle.