Mitch Rales is one of the few D.C. area residents to make Forbes’ list of the world’s billionaires, coming in at No. 323. He and brother Steven (who’s number 336) made their billions founding Danaher Corp., a tech and manufacturing conglomerate. Mitch has chosen to use some of his riches to invest in the arts. He sits on the boards of the National Gallery of Art and the Hirshhorn, and in 2006, he opened Glenstone, a private museum next to his home in Potomac, Md. Getting a look at its holdings isn’t easy: The collection is viewable by appointment only, and the museum doesn’t respond to requests from the media.