  We study thin self-assembled columns constrained to lie on a curved, rigid
substrate. The curvature presents no local obstruction to equally spaced
columns in contrast to curved crystals for which the crystalline bonds are
frustrated. Instead, the vanishing compressional strain of the columns implies
that their normals lie on geodesics which converge (diverge) in regions of
positive (negative) Gaussian curvature, in analogy to the focussing of light
rays by a lens. We show that the out of plane bending of the cylinders acts as
an effective ordering field.
