  We show that the Second Derivative (SD) technique of the absorption spectra
of Rb atomic vapours confined in a nanocell with a thickness $\ell=
\lambda/2=398$ nm allows to achieve close to Doppler-free spectroscopy. Narrow
linewidth and linearity of the SD signal response with respect to transition
probabilities allow us to study separately, in an external transverse magnetic
field (0.6 to 4 kG), a big number of the atomic transitions of $^{85}$Rb and
$^{87}$Rb atoms. Atomic transitions $|F_g,0\rangle \rightarrow
|F_e=F_g,0'\rangle$, for which the dipole moment is null at zero magnetic field
(so-called magnetically-induced transitions), show a gigantic increase in
probability with increasing magnetic field. When a magnetic field is applied on
the vapour, we show the possibility of forming a dark resonance on these
transitions by adding a coupling laser. We hence demonstrate a five-fold
increase in the transmission of the probe radiation when the coupling laser is
on. Theoretical calculations are in a very good agreement with experimental
results.
