Table 
in Appendix gives sensitivity values for ISIS and FOS-2
measured from wide slit
observations of spectrophotometric standards,
in the case of ISIS with the low dispersion (158 line/mm) gratings.
These measurements give the overall performance for the entire system,
including atmosphere, telescope, spectrograph and detectors. The
performance is expressed as a magnitude (AB magnitude in Oke's
system; see for instance Oke, Astrophys. J. suppl. 27, 21,
1974) which gives a measured count rate of 1 photon/second/Å
at the detector.