  We introduce an observational tool based on visibility nulls in optical
spectro-interferometry fringe data to probe the structure of stellar
atmospheres. In a preliminary demonstration, we use both Navy Precision Optical
Interferometer (NPOI) data and stellar atmosphere models to show that this tool
can be used, for example, to investigate limb darkening.
  Using bootstrapping with either multiple linked baselines or multiple
wavelengths in optical and infrared spectro-interferometric observations of
stars makes it possible to measure the spatial frequency $u_0$ at which the
real part of the fringe visibility ${\rm Re}(V)$ vanishes. That spatial
frequency is determined by $u_0 = B_\perp/\lambda_0$, where $B_\perp$ is the
projected baseline length, and $\lambda_0$ is the wavelength at which the null
is observed. Since $B_\perp$ changes with the Earth's rotation, $\lambda_0$
also changes. If $u_0$ is constant with wavelength, $\lambda_0$ varies in
direct proportion to $B_\perp$. Any departure from that proportionality
indicates that the brightness distribution across the stellar disk varies with
wavelength via variations in limb darkening, in the angular size of the disk,
or both.
  In this paper, we introduce the use of variations of $u_0$ with $\lambda$ as
a means of probing the structure of stellar atmospheres. Using the equivalent
uniform disk diameter $\theta_{\rm UD, 0}(\lambda_0)$, given by $\theta_{\rm
UD, 0} = 1.22/u_0(\lambda_0)$, as a convenient and intuitive parameterization
of $u_0(\lambda_0)$, we demonstrate this concept by using model atmospheres to
calculate the brightness distribution for $\nu$ Ophiuchi and predict
$\theta_{\rm UD, 0}(\lambda_0)$, and then comparing the predictions to
coherently averaged data from observations taken with the NPOI.
