  We develop a scaling theory for the finite-size critical behavior of the
microcanonical entropy (density of states) of a system with a
critically-divergent heat capacity. The link between the microcanonical entropy
and the canonical energy distribution is exploited to establish the former, and
corroborate its predicted scaling form, in the case of the 3d Ising
universality class. We show that the scaling behavior emerges clearly when one
accounts for the effects of the negative background constant contribution to
the canonical critical specific heat. We show that this same constant plays a
significant role in determining the observed differences between the canonical
and microcanonical specific heats of systems of finite size, in the critical
region.
