  It is shown that the Anderson transition (AT) in 3d obeys a two-parameter
scaling law, derived from a pair of anisotropic scaling transformations, and
corresponding critical exponents and scaling function calculated, using a
high-precision numerical finite-size scaling study of the smallest Lyapunov
exponent of quasi-1d systems of rectangular cross-section of L times l atoms in
the limit of infinite L and l < L, for x=l/L ranging from 1/30 to 1/4. The
second parameter is x, and there are two singularities: apart from the
two-parameter scaling describing AT for x>0, corrections to scaling due to the
irrelevant scaling field diverge when x->0, and the corresponding crossover
length scale is also estimated. Furthermore, results suggest that the
signatures of the AT in 3d should be present also in 2d strongly localized
regime.
