At optical wavelengths we know that quasars evolve such
that at a given luminosity the space density increases as
redshift increases up to a redshift of 2-3, but we do not
know whether this change is due to density or luminosity evolution.

We intend to do photometry on a sample of bright quasars
at a variety of wavelengths, at 7.3, 11.5, 25, 60, and 100
microns. With these data we will determine the spectral energy
distributions. This sample of quasars has been carefully
constructed such that it spans a wide range in redshift and
a narrow range in optical luminosity, in order to eliminate the
usual luminosity-redshift degeneracy inherent in most quasar
samples.

With this sample of quasars
we will be able to see if the IR S.E.D.s of the quasars also
change with redshift and we can test the hypothesis that the
observed evolution in the optical is due to the evolution in
the covering factor of dust.
The quasars have been selected from Hewitt and Burbidge, and the
recently completed Edinburgh Multicolour Quasar survey. None
of the sources are in the Central Program.

ISO data is essential for this experiment as we need to measure
the S.E.D.s of these sources in the IR and over as wide a wave-
length range as possible.

Note- two diagrams are included in the paper version of this proposal
but are excluded from the electronic version.