We present a variational calculation of the envelope wave function of
an exciton inside a cube, in the effective mass approximation, with on
e parameter which plays the part of a Bohr radius. This geometry allow
s one to reduce the sixfold integrals to threefold integrals which sho
rtens tremendously computer calculations. This wave function is used t
o obtain the exchange energy and the oscillator strength. The limiting
cases (large or small cubes) are recovered. We show that recent exper
imental results in porous silicon can be explained without further hyp
othesis.