NEW YORK • In a ballet season of six or eight weeks, some artists change fast - and the proscenium arch becomes a magnifying glass through which, before the viewers' eyes, a dancer becomes a heroine.

So it was this spring with two young American Ballet Theatre principals who have ascended through the company's ranks - Isabella Boylston and Devon Teuscher.

The transformation in them is not yet absolute. And who knows? Perhaps they will lose what they have gained. Both perform at the Vail International Dance Festival next month, making debuts in new roles.

Certainly, there were weeks this year in which they seemed to have grown immeasurably, weeks that built upon others the year before.

These two are interestingly - and increasingly - unalike.

Boylston, 31, is an engaging, winning personality; Teuscher, 29, is movingly impersonal, dancing objectively as if serving the choreography rather than herself. And in them, you start to see the range of expression ballet itself can have.

Though ballet's formal vocabulary is capable of multiple speeds, classroom tradition divides it into two: adagio and allegro, slow (and sustained) or fast (and intricate).

Both Teuscher and Boylston blaze through the 32 fouette turns of Swan Lake and take to the air impressively; but Teuscher's natural domain seems adagio, Boylston's, allegro.

Teuscher's dancing draws attention to shape, contours, sequences; Boylston's to sparkling footwork and effervescent jumps.

Boylston is bubbly, exuberant, finely musical. She has an exceptional leap, an ebullient soubrette quality and a touch of laughter in her movement. She is in many ways already a complete star - with 240,000 followers on Instagram.

She has also become a maker and shaker in the ballet world. She joined Ballet Theatre's Studio company in 2005 and became a principal in 2014.

Recently, when actress Jennifer Lawrence played a Russian ballerina in the 2018 film Red Sparrow - the choreography was by Justin Peck - Boylston was her well-publicised dance double.

This July and last, she has brought ballet to her home town, with the festival Ballet Sun Valley, in Idaho, to which she invites European and American artists.

Teuscher, who joined the Studio Company in 2006, is firmly dramatic, coolly decisive, enigmatic, often mysteriously calm.

When Alexei Ratmansky made Serenade After Plato's Symposium (2016), he cast Teuscher as its sole woman: It was implied that she was the seer, Diotima, whom Socrates consults about the nature of love - the sage who knows the mysteries the men hope to understand.

Last year, Teuscher became a principal. She is not yet fully a star; she does not invariably give off light.

She is, however, naturally, serenely grand. Though the word "ballerina" really means only "dancing woman" in its original Italian, it has been inflated by over a century of Russian ballet mystique to acquire multiple connotations of authority, many of which Teuscher seems to exemplify, quietly but surely.

This spring, her account of Myrta, the spectral queen of the otherworldly siren wilis in Giselle, had more luminescence than some of the ballerinas playing Giselle.

In the season's second week, she succeeded the illustrious Alessandra Ferri as the heroine of Wayne McGregor's new AfteRite.

Remarkably, she showed much the same capacity for poignancy as Ferri, whether protesting angrily or merely walking slowly.

After that came a gorgeous debut as Nikiya in La Bayadere. Nikiya, an Indian temple dancer, has a glorious slow entrance with a veil over her face - glorious because the High Brahmin then tears the veil away, revealing her face like a work of art.

Teuscher gave the moment real impact: Her cheekbones, large eyes and beautifully held head register powerfully through the Metropolitan Opera House as few others do.

And she moves the way she looks: Her line has amplitude; her phrases unfold smoothly, like thick cream.

Though these woman share a number of roles, their differences turn each ballet into a wholly separate form of drama. Those contrasts are just as evident outside the theatre.

Boylston uses social media with the same glee with which she takes the stage.

Teuscher, by contrast, is more guarded. Not invariably: This spring, she and fellow principal Cory Stearns let it be known that they were a couple.

Onstage, Boylston gives the impression she is vibrating even when she is immobile. With Teuscher, viewers sense that still waters run deep.

This spring, you could feel the Ballet Theatre audience's idea of womanhood widening as each of these two heroines became more fully herself.

NYTIMES