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The translucency and colour of a composite resin are characterized by two wavelength-dependent parameters: the absorption coefficient, K and the scattering coefficient, S. These parameters were calculated according to the Kubelka-Munk (1931) equations for two commercial resins. The calculations were based on reflectance spectra of samples of different thicknesses which were placed on a black and a white background. Employing the values of these parameters, calculated spectra were obtained of samples on the same as well as different backgrounds. The difference between the experimental and the calculated spectra was evaluated in terms of Friele-MacAdam-Chickering (Hemmendinger, 1970) colour differences. The Munsell colour values, corresponding to the spectra were evaluated. It was found that the colour of a resin sample of a specified thickness which is placed on a particular background may be predicted to a fair degree of accuracy from calculations employing the K and S values of the resin and the reflectance spectrum of the background. It has been demonstrated that illuminating the sample at 45 degrees instead of using diffuse illumination, affected the results only to a small extent. The effect of internal reflections at the resin surface on the relfectivity on the sample is discussed. An expression has been derived for computing the reflectivity of a sample on any background from its transmittance and reflectivity on a completely absorbing background.

Potential energy curves of ground and excited states of tetra halomethanes and the negative ionsNegative-ion breakdown curves were obtained at low (0-1 eV) energies for SFe and a number of fluorocarbons. The dissociation rate constants, as a function of internal energy of the parent negative ions, were computed for the processes observed and used to estimate kinetic shifts and thermochemical appearance potentials, at OaK, for the fragments. The results (e.g., metastable curve widths and temperature effects) were found to be in agreement with the quasiequilibrium theory (QET) of mass spectra. It is predicted that most of the parent negative ions studied (e.g., C7F 14 -from perfluoromethy1cyc1ohexane) would not fragment if formed by the capture of a thermal electron by the vibrationally cold molecule.

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Non-precious metal crowns were prepared for natural teeth which were ground to a taper angle of 5 degrees. The movement of the crowns on teeth during repeated try-on seating was measured with a micrometer. Increasing the applied load from 2 to 10 kg resulted in an average crown movement of 63 micron. For most samples this movement continued when try-on was repeated. Casting irregularities were found to create furrows on some regions of the axial tooth surface during try-on seating. The diameter of the ADA specification consistency disk for zinc phosphate cement was found to depend strongly on the P:L ratio and the ambient temperature. The castings were cemented with zinc phosphate cement of controlled consistency. Cementation with a load of 10 kg gave rise to an average crown elevation of 54 micron and a range of measurements between -8 and 113 micron. The effective minimum cement thickness corresponding to this average was calculated to be 4.7 micron. Small negative values of elevation were attributed to deepening of tooth furrows during cementation. An analysis is given of data from the literature, relevant to the cement flow and irregularities in cement thickness.