  Metis is the first solar coronagraph designed for a space mission capable of
performing simultaneous imaging of the off-limb solar corona in both visible
and UV light. The observations obtained with Metis aboard the Solar Orbiter
ESA-NASA observatory will enable us to diagnose, with unprecedented temporal
coverage and spatial resolution, the structures and dynamics of the full corona
from 1.7 $R_\odot$ to about 9 $R_\odot$. Due to the uniqueness of the Solar
Orbiter mission profile, Metis will be able to observe the solar corona from a
close vantage point (down to 0.28 AU), achieving out-of-ecliptic views with the
increase of the orbit inclination over time. Moreover, observations near
perihelion, during the phase of lower rotational velocity of the solar surface
relative to the spacecraft, will allow longer-term studies of the coronal
features. Thanks to a novel occultation design and a combination of a UV
interference coating of the mirrors and a spectral bandpass filter, Metis
images the solar corona simultaneously in the visible light band, between 580
and 640 nm, and in the UV H I Lyman-{\alpha} line at 121.6 nm. The coronal
images in both the UV Lyman-{\alpha} and polarised visible light are obtained
at high spatial resolution with a spatial scale down to about 2000 km and 15000
km at perihelion, in the cases of the visible and UV light, respectively. A
temporal resolution down to 1 second can be achieved when observing coronal
fluctuations in visible light. The Metis measurements will allow for complete
characterisation of the main physical parameters and dynamics of the electron
and neutral hydrogen/proton plasma components of the corona in the region where
the solar wind undergoes acceleration and where the onset and initial
propagation of coronal mass ejections take place, thus significantly improving
our understanding of the region connecting the Sun to the heliosphere.
