  Using simultaneous observations of the MDI and EIT instruments on board the
SoHO spacecraft, we determined in flight the plate scale of the EIT. We found a
value of 2.629+-0.001 arc seconds per pixel, in fair agreement with the
2.627+-0.001 arc seconds per pixel value deduced from recent laboratory
measurements of the focal length, and much higher by 7 sigma than the 2.622 arc
seconds per pixel value of the pre-flight calibrations. The plate scale is
found to be constant across the field of view, confirming the negligible
distortion level predicted by the theoretical models of the EIT. Furthermore,
the 2 sigma difference between our results and the latest laboratory
measurements, although statistically small, may confirm a recent work
suggesting that the solar photospheric radius may be 0.5 Mm lower than the
classically adopted value of 695.99 Mm.
