We present the far-UV spectrum of the quasar HS~1700+6416 taken with FUSE .
This QSO provides the second line of sight with the He ii absorption resolved into a Ly \alpha forest structure .
Since HS 1700+6416 is slightly less redshifted ( z _ { \mathrm { em } } = 2.72 ) than HE~2347-4342 , we only probe the post-reionization phase of He ii , seen in the evolution of the He ii opacity , which is consistent with a simple power law .
The He ii /H i ratio \eta is estimated using a line profile-fitting procedure and an apparent optical depth approach , respectively .
The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H _ { 2 } .
About 27 % of the \eta values are affected by metal line absorption .
In order to investigate the applicability of the analysis methods , we create simple artificial spectra based on the statistical properties of the H i Ly \alpha forest .
The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only ( 0.01 \leq \tau _ { { H \textsc { i } } } \leq 0.1 and 12.0 \leq \log N _ { { H \textsc { i } } } \leq 13.0 , respectively ) .
The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits .
Furthermore , applying the methods to the unrestricted data set may mimic a correlation between the { He \textsc { ii } } / { H \textsc { i } } ratio and the strength of the H i absorption .
For the restricted data samples a scatter of 10 - 15 % in \eta would be expected even if the underlying value is constant .
The observed scatter is significantly larger than expected , indicating that the intergalactic radiation background is indeed fluctuating .
In the redshift range 2.58 < z < 2.72 , where the data quality is best , we find \eta \sim 100 , suggesting a contribution of soft sources like galaxies to the UV background .