  We investigate the structure of the solar chromosphere and transition region
using full Sun images obtained with the Extreme Ultraviolet Imaging Telescope
(EIT) aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. The limb
seen in the EIT coronal images (taken in lines of Fe IX/X at 171 \AA, Fe XII at
195 \AA and Fe XV at 284 \AA) is an absorption limb predicted by models to
occur at the top of the chromosphere where the density of neutral hydrogen
becomes significant ($\sim10^{10}$ cm$^{-3}$). The transition-region limb seen
in He II 304 \AA images is an emission limb. We find: (1) the limb is higher at
the poles than at the equator both in the coronal images (by 1300 $\pm$ 650 km)
and the 304 \AA images (by 3500 $\pm$ 120 0 km); and (2) the 304 \AA limb is
significantly higher than the limb in the coronal images. The height difference
is 3100 $\pm$ 1200 km at the equator, and 6600 $\pm$ 1200 km at the poles. We
suggest that the elevation of the 304 \AA limb above the limb in the coronal
images may be due to the upper surface of the chromosphere being bumpy,
possibly because of the presence of spicules. The polar extension is consistent
with a reduced heat input to the chromosphere in the polar coronal holes
compared with the quiet--Sun atmosphere at the equator.
