Seeing Leonardo's "Lady with an Ermine" in the flesh for the first time is a real "Mona Lisa" moment: a mental image constructed from dozens of reproductions is brutally confronted with its original. The memories of CMYK clones pale in front of this "Portrait of Cecilia Gallerani," as the picture is also known. In her gilded frame, the subject is breathtakingly alive, so present, so real — we've just caught the teenage mistress of Leonardo's patron Ludovico Sforza listening, perhaps, to her lover's endless political schemes. She is at once respectful, amused, and slightly bored as she scratches the neck of her favorite pet. In just a moment, she'll walk away, the heavy fabric of her robe sweeping the marble floor of her Milanese palazzo.

One could easily talk about the symbolism of the white beast, and its significance for both the sitter and Sforza, but the overwhelming reaction this painting commands is a visceral one. "It's very hard not to fall in love with Cecilia," said curator Luke Syson, "and that's what Leonardo intended." Painted between 1452 and 1519, "Lady with an Ermine" is arguably the jewel of the National Gallery's "Leonardo da Vinci: Painter at the Court of Milan." This is no small feat, as the show counts several other masterpieces, including both versions of the "Virgin of the Rocks." But the portrait of Cecilia Gallerani shines through. It stands as the purest expression of Leonardo's attempt to capture the world, before, as Syson argues, the artist moved on to create ideal versions of it instead.

This trajectory is the conceptual thread binding this show, which is specifically dedicated to the paintings Leonardo made during his time in Milan at the service of the city's duke Sforza, between 1492 and 1499. The exhibition — the largest display of Leonardo's surviving paintings to date — opens with an ink-on-paper cross-section of a man's head. Titled "The Ventricles of the Brain and the Layers of the Scalp" (about 1490-4), this picture is an understated yet striking illustration of the artist's obsession with the visual, and its impact on human psyche. Here, the eyeball leads directly to three small chambers that, Leonardo thought, were responsible for data processing and collecting, as well as for the human soul, the imagination, and memory. "Leonardo experienced the world through the eye and presented it to the eye," said Syson. "He believed that a perfect beauty could speak to the soul."

Cecilia's counterpart, in life as in painting, is the 1493-4's "Portrait of a Woman," also known as "The Belle Ferronière." The model is unknown but it is thought be another mistress of Ludovico, or possibly his wife, the duchess Beatrice d'Este. Like Cecilia, she's pulsating with life, but she is also kept at a distance from the viewers by means of a little parapet that prevents her from entering our dimension, and with her stylized features: the flawless oval of her head, and the geometric, diagram-like patterns of her clothing. The portrait of Cecilia celebrates one individual, "The Belle Ferronière" theorizes on beauty and womanhood.

Leonardo is known to have transformed portraiture: he famously brought to Milanese painting — at the time of his arrival still stuck in a medieval style that showed the sitters' profiles — a Florentine liveliness and a form of realism religiously copied by his followers. But this expertly curated show demonstrates that the "Leonardo revolution" involved much more than shifting models from profile to three quarter view. The artist's oscillations between the real and the fantasized mirror the vagaries of the human condition.