In this proposal, more time is being requested for RRUBIN.PN.  We also propose
observations that extend RRUBIN.PN to 2 more PNs that were part of the original
"Spring" program and will become accessible during the extended mission.
Recent work has shown that nebular abundances can be in error by as much as a
factor of ~5, possibly due to unexplained temperature fluctuations (t^2).  We
propose to use ISO IR spectroscopy, cospatial optical/UV spectra, and our
theoretical photoionization codes, to understand the physical conditions on the
microscopic scale within a few PNs.  This takes advantage of heretofore
unobservable IR lines, which are especially useful because of their weak depend-
ence on electron temperature (Te) and extinction.  Proposed ISO observations
include 7 pairs of electron density (Ne) sensitive line ratios of different
ionic species (Ar++, Ar+4, Ne++, Ne+4, O++, S++, and Mg+4) - all with 5-lowest
energy levels (^3P_0,1,2;^1D_2;^1S_0).  With these data as well as additional
Ne-sensitive lines in the optical-UV, we will examine the density and ioniza-
tion structure of the PNs in greater detail than has been previously possible.
Cospatial observations from the ground or available from IUE of the "nebular"
(from ^1D_2) and "auroral" (from ^1S_0) lines together with our ISO ground-
state pair will permit the measurement of Te and t^2.  The measurement of t^2
for most of these ions has not been possible previously, and will provide
information on how t^2 depends on the ionization level of the gas.
Our ultimate goal is to significantly improve the gas-phase abundance estimates
for all heavy elements observed.  For refractory elements, we will determine
abundances as a function of ionization stage (nebular position).  This will
help determine the extent of grain destruction within the nebula and the effects
on the thermal balance of the gas.  All of these goals have as a common under-
pinning - understanding the conditions within the nebulae and the physical
origin of the inferred Te variations, and finding a prescription for
determining reliable nebular abundances - fundamental to understanding the
chemical evolution of our Galaxy.