You know the comedy-tragedy masks, with the muse of tragedy, Melpomene, and the muse of comedy, Thalia. The lesser seen third mask is what Walter Harte called "the slatten-muse, Indifference" (Eulogius or, the Charitable Mason, 1767). (We assume that slatten derives from the stone slab of masonry called a slatt, so as to imply stony-faced apathy.)
Our illustration is from Emerson's 1922 yearbook.