  Using positional data from video-microscopy of a two-dimensional colloidal
system and from simulations of hard discs we determine the
wave-vector-dependent normal mode spring constants in the supercooled fluid and
glassy state, respectively. The emergence of rigidity and the existence of a
displacement field in amorphous solids is clarified. Continuum elastic theory
is used in the limit of long wavelengths to analyze the bulk and shear modulus
of this amorphous system as a function of temperature. The onset of a finite
static shear modulus upon cooling marks the fluid/solid transition. This
provides an opportunity to determine the glass transition temperature $T_g$ in
an intuitive and precise way.
