===================================================================== ==> In this proposal, more time is being requested for MMEIXNER.PROP_TDH ===================================================================== We propose to obtain linear scans of the thermal emission from dust in two planetary nebulae (IC 418 and NGC 7293) and three proto-planetary nebulae (GL 618, GL 915 and IRAS18184-1623). Our strategy is to use the the ISOPHOT FIR arrays, C100, and C200 to image these nebulae at wavelengths greater than 100 um. The two goals of these observations is (1) to determine the history of mass loss as imprinted on the circumstellar dust envelopes in these PPNe and PNe, and (2) to measure the total mass of the circumstellar dust from which we can infer a lower limit to the initial mass of the progenitor main sequence star. In particular, the radial profiles of dust density derived from these observations will test current theories of episodic mass loss and accelerated mass loss. Understanding this mass loss process is a key problem within astrophysics because most of interstellar dust and gas originates from these stellar sources. ISO provides the necessary sensitivity and longer wavelength coverage to detect the 35K thermal dust halos surrounding PPNe and PNe.