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1703 linear superfluid dissipative force in analyzing their isothermal flow experiments.Their force varied linearly with the excess superfluid velocity above a critical velocity v, , that depended on the width of their slits [F"=P(v,v, , )j. Below v, " the superfluid flowed with no dissipation. Their P coefficient was temperature dependent and had a value of 55 gm/cms sec at 1.7 'K for a 2. 29pwide slit. (Their results for a 0. 31p, -wide slit did not yield significantly different values of P. )It is evident that a linear superfluid force may be used to explain our anomalous results but the coefficient representing the strength of this force must be larger than that found by Winkel et al. ' for heat flow through a 1. 5p, -wide slit or that resulting from the narrow-slit isothermal-flow experiments of Keller and Hammel.~~Further, it is not clear why this extra dissipation was not observed with the 3.6-cm-length 8. 7p. -diameter channels. More measurements are needed before this aspect of heat flow under small grad 7 in small channels can be discussed more quantitatively.*Work based on part of a thesis that was submitted by E. C. Alcaraz in partial fulfillment of the requirements for the degree of Doctor The transition from ambipolar to free diffusion for He&' in a helium afterglow has been observed with a mass spectrometer.The theory of Allis and Rose pred'cts the general features of the observed behavior.Since the time of Langmuir, there have been many theoretical attempts to characterize the spacecharge fields in a bounded ionized medium. In the past, both experimental measurements and the theory have been concerned almost exclusively with the electrons. ' Recently, studies showing the transition from electron-ion ambipolar diffusion to negative-ion-positive -ion ambipolar diffusion have been published. & ' The complete transition from electron-ion ambipolar diffusion to free diffusion for the positive ions has not as yet been reported, although part of the transition region may have been observed.The present paper reports results of experiments which show the transition from ambipolar to free diffusion for He&' in helium. It will be shown here that the qualitative behavior of the ion afterglow decay can be predicted by extending the analysis of Allis and Rose' (hereafter desig-

An analysis of the lateral shearing interferometer is given for laser beams. The results enable a simple determination of the local radius of curvature of the wavefront. In addition, the presence of phase distortion in the beam may be ascertained.