Excerpt from The Elements of Non-Euclidean Geometry
The heroic age of non-euclidean geometry is passed. It is long since the days when Lobachevsky timidly referred to his system as an "imaginary geometry", and the new subject appeared as a Это и многое другое вы найдете в книге The Elements of Non-Euclidean Geometry (Classic Reprint) (Julian Lowell Coolidge)