  We have succeeded in developing hydrophobic silica aerogels over a wide range
of densities (i.e. refractive indices). A pinhole drying method was invented to
make possible producing highly transparent aerogels with entirely new region of
refractive indices of 1.06-1.26. Obtained aerogels are more transparent than
conventional ones, and the refractive index is well controlled in the pinhole
drying process. A test beam experiment was carried out in order to evaluate the
performance of the pinhole-dried aerogels as a Cherenkov radiator. A clear
Cherenkov ring was successfully observed by a ring imaging Cherenkov counter.
We also developed monolithic and hydrophobic aerogels with a density of 0.01
g/cm^3 (a low refractive index of 1.0026) as a cosmic dust capturer for the
first time. Consequently, aerogels with any refractive indices between 1.0026
and 1.26 can be produced freely.
