  The ANTARES neutrino telescope is a large photomultiplier array designed to
detect neutrino-induced upward-going muons by their Cherenkov radiation.
Understanding the absorption and scattering of light in the deep Mediterranean
is fundamental to optimising the design and performance of the detector. This
paper presents measurements of blue and UV light transmission at the ANTARES
site taken between 1997 and 2000. The derived values for the scattering length
and the angular distribution of particulate scattering were found to be highly
correlated, and results are therefore presented in terms of an absorption
length lambda_abs and an effective scattering length lambda_sct^eff. The values
for blue (UV) light are found to be lambda_abs ~ 60(26) m, lambda_sct^eff ~
265(122) m, with significant (15%) time variability. Finally, the results of
ANTARES simulations showing the effect of these water properties on the
anticipated performance of the detector are presented.
