In spectra of chemically peculiar (CT) stars the weak lines originate at very different levels on the both sides of the Balmer jump (λB = 3646 A): τ ≈ 0.01 at λ > 3646 Å and τ ≈ 0.5–1.0, when λ > 3646 Å (Khokhlova, 1978)
To estimate the radial gradient of the magnetic field, we have observed two CP-stars β CrB and α2CVn. from 1979 to 1984 on the 6-meter telescope with an achromatic circular polarization analyzer (Glagolevskij et al.,1978). Compared with classical Zeeman analyzers, whose working wavelength band is only 300 Å, the achromatic analyzer covers a wide spectral region, enabling many more lines to be measured.
A combination of camera and grating was chosen so as record the 3300 - 4000 Å region all on the same plate at the 6.7 A/mm dispersion. Since the lines used to measure the field on either side of λ 3646 Å are exposed simultaneously on a single plate, one avoids many of the systematic errors arising from various positional, photometric and polarization effects, all these should influence the short and long wave ends of the spectrogramm identically.