Two Nike-Apache payloads were launched into noctilucent cloud (NLC) displays from Esrange, Sweden in the summer of 1971 and two in the summer of 1973. A two-color photopolarimeter was part of the multiexperiment payload that included the Pandora collector of Dudley Observatory. The absolute brightness and degree of polarization are used to determine the size and number density of NLC particles. The Pandora collection size distribution and two arbitrary size distributions of ice, iron, and nickel are compared with the observations. The observed brightness and polarization suggests that the particles are principally less than 0.13-t•m radius with number density of 106 particles/m 3. The difference between the optical and the collection results suggests that either a large number of particles are undetected by collection or a significant number of the particles are the result of nucleation on complex ions. Two Nike-Apache payloads were launched at Esrange, Sweden, into noctilucent cloud (NLC) displays on the nights of July 31 and August 1, 1971, and two on the night of August 1, 1973. A two-color photopolarimeter was part of a multiexperiment payload consisting of the Pandora collector from Dudley Observatory [Hallgren et al., 1973], a plasma impact detector from Max Planck Institut ffir Kernphysi•k at Heidelberg, Germany [Rauser and Fechtig, 1973], and an acoustic particle impact detector from Lund Observatory in ß Lund, Sweden [Lindblad et al., 1973]. In 1971 the first payload (14.468) observed an NLC layer on ascent and descent. The second payload (14.469) indicated no NLC layer, although a polarimeter of Witt et al. [1971] launched 30 rain later observed a distinct NLC layer. This dynamic nature of the NLC observations was further confirmed when in 1973 the two payloads (14.515 and 14.516), launched 30 min apart, observed a Significant brightness difference. INSTRUMENT The optics consist of a single 5.5-cm-diameter lens, a Polaroid, and a bifurcated light guide (Figure 1). Two interference filters centered at 5400 and 4100 A with a 50-A halt'power band pass exclude intense atmospheric emission lines. The electronics for each color consists of a photomultiplier operating at 106 gain and a 6-decade current electrometer with a 0-to 5-V output signal. The instrument is axially mounted, and a 4-to 6-rps Payload spin rotates the Polaroid. The instruments were basically identical in both series of flights. The 1971 photopolarimeters had a 1 ø full angle field of view and a minimum detectivity of 1 x 10-•' W cm-'" sr-• A-• For the 1973 flights a field of view of 2.5 ø and low dark current photomultipliers improved the signal to noise ratio at the NLC and lowered the minimum detectivity to 4 X 10 -•4 W cm-'" sr-' A-•. The sunshield and optical system reduced stray light to less than 4 X 10-TM W cm-'" sr-x A-• up to a solar aspect angle of 77 ø in 1971 and 65ø in 1973 [Leinert; 1971]. The brightness response and polarization characteristics of the instrument were measured before and after each flight. In addition to compo...