“Silhouettes are reductions,” the artist Kara Walker said in 2014, “and racial stereotypes are also reductions of actual human beings.” Walker’s large-scale silhouettes, which powerfully confront America’s dehumanizing history of racism and slavery, have long haunted museumgoers. In this virtual exhibition two important works are on display: a short film from 2009 called Bureau of Refugees, Freedmen and Abandoned Lands: Six Miles from Springfield on the Franklin Road, and the 2013 silhouette installation The Sovereign Citizens Sesquicentennial Civil War Celebration. Both are shocking depictions of calamity and injustice. —E.C.