  Aims: We present J, H, K interferometry with a spectral resolution of 35 for
the Mira variable S Orionis. We aim at measuring the diameter variation as a
function of wavelength that is expected due to molecular layers lying above the
continuum-forming photosphere.
  Methods: Visibility data of S Ori were obtained at phase 0.78 with the
VLTI/AMBER instrument using the fringe tracker FINITO at 29 spectral channels
between 1.29 and 2.32 mu. Apparent uniform disk (UD) diameters were computed
for each spectral channel. In addition, the visibility data were directly
compared to predictions by recent self-excited dynamic model atmospheres.
  Results: S Ori shows significant variations in the visibility values as a
function of spectral channel that can only be described by a clear variation in
the apparent angular size with wavelength. The closure phase values are close
to zero at all spectral channels, indicating the absence of asymmetric
intensity features. The apparent UD angular diameter is smallest at about 1.3
and 1.7 mu and increases by a factor of ~1.4 around 2.0 mu. The minimum UD
angular diameter is 8.1 pm 0.5 mas, corresponding to ~420 R_sun. The S Ori
visibility data and the apparent UD variations can be explained reasonably well
by a dynamic atmosphere model that includes molecular layers.
  Conclusions: The measured visibility and UD diameter variations with
wavelength resemble and generally confirm the predictions by recent dynamic
model atmospheres. [abridged]
