  Solid interfaces have intrinsic elasticity. However, in most experiments,
this is obscured by bulk stresses. Through microscopic observations of the
contact-line geometry of a partially wetting droplet on an anisotropically
stretched substrate, we measure two surface-elastic constants that quantify the
linear dependence of the surface stress of a soft polymer gel on its strain.
With these two parameters, one can predict surface stresses for general
deformations of the material in the linear-elastic limit.
