Widmann, a Swiss poet, describes Brahms’ performing at the piano:

The broad leonine chest, the Herculean shoulders, the mighty head which the player sometimes threw back with an energetic jerk, the pensive, handsome brow that seemed to radiate an inner illumination, and the Germanic eyes which scintillated with a wondrous fire between their fair lashes – all this betrayed an artistic personality that seemed charged to the very finger tips with the fluid of genius.  There was, too, something confidently victorious in the countenance, the radiant serenity of a mind that is happy in the exercise of its art.

Source: Goss, Madeleine & Schauffler, Robert (1943) Brahms The Master. New York: Henry Holt and Company, p.210.