For this work, Degas took his drawing of the dancer in white and added pieces of paper to create a larger composition, incorporating another dancer and the large expanse of floor before her. The artist left notations behind at right—perhaps measurements of the added strips. Remarkable here is the range of effects he achieves with pastel. Small, linear strokes describe the glints of light on the dancers’ hair, broader strokes the opaque outer skirts. For the puffy underskirt, Degas rubbed the medium, with either his hand or a rag. The density or diffusion of the pigment coincides with the way light behaves on each surface depicted. He also crushed his pastels and added liquid, creating a thin...