  Despite the influence of magnetic fields on the structure and evolution of
stars is largely demonstrated from the theoretical point of view, their
observational evidence in non-degenerated stars is still rather scanty and
mainly circumscribed to bright objects (V<10). Stellar magnetic fields are
commonly measured on the basis of circular spectropolarimetry at high/middle
resolution across the profile of metal lines. The present sensitivity of
telescopes and spectrographs makes this still an almost prohibitive method for
faint stars. In principle, stellar magnetic fields can be also measured on the
basis of low resolution spectropolarimetry, with very important results
obtained at the 8 m ESO telescopes with FORS1. The trade off between S/N and
spectral resolution in measuring stellar magnetic fields justify an attempt,
here presented, to perform these measurements at the 4.5 m William Herschel
Telescope. HD3360, one of the stars with the weakest known magnetic field, and
the magnetic chemically peculiar stars HD10783, HD74521 and HD201601 have been
observed with the spectropolarimeter ISIS in the 3785-4480 A range. Measured
stellar magnetic fields, from Stokes I and V spectra with S/N > 600, show an
internal error of <50 G selecting the whole interval and <200 G within a Balmer
line. Ripples in the Stokes V spectra of HD3360 result in an instrumental
positive magnetic field certainly not larger than 80 G.
