  GRB 990123 is the most luminous event detected so far, and in an important
paper, Hjorth et al. (1999) reported an upper limit on the degree of linear
polarization of the optical afterglow for this burst (P <2.3%). One of the
interprtations for this small value of P was that the emission was probably due
a relativistic jet with ordered magnetic field, and the viewing angle in the
lab frame theta' <= Gamma^{-1}, where Gamma is the bulk Lorentz factor of the
jet at the time of the optical emission. We point out that this conclusion
resulted from a confusion between the angles measured in the lab frame and in
the plasma rest frame. For ordered magnetic field, one would actually obtain a
large value of P because the above mentioned angle would correspond to a very
large angle, theta <= \pi/2, in the plasma rest frame. And this is probably the
case with the blazars. On the other hand, it is indeed possible to have P ~ 0
if the magnetic field of GRB 990123 was completely chaotic and the viewing
angle was considerably smaller than the semi-angle of the jet.
