  A new system, consisting of a double channel Fabry Perot etalon and laser
diodes emitting around 780 nm is described and proposed to be used for air
refractive index measurements. The principle of this refractometer is based on
frequency measurements between optical laser sources. It permits
quasi-instantaneous measurement with a resolution better than and uncertainty
in the range. Some preliminary results on the stability of this system and the
measurements of the refractive index of air with this apparatus are presented.
The first measurements of the index of air at 780 nm are, within an
experimental uncertainty of the order of, in agreement with the predicted
values by the so-called revised Edl\'en equations. This result is to the best
of our knowledge the first to extend to the near infra-red the validity of the
revised Edl\'en equation derived for the wavelength range 350- 650 nm.

Optics