In view of the great demand for travel services in North America at the time of the Congress it is strongly recommended that Congress participants make immediate arrangements for their transportation. Living Accommodation Provisional reservation of rooms in Montreal has already been made. These will be allotted in the order of receipt of the registration forms which accompany the general information booklet. Because accommodation in Montreal hotels and motels is much in demand during the summer months it is strongly recommended that these forms be sent to the nearest American Express office or correspondent as early as possible, and not later than 15 February 1957, to ensure the accommodation desired. Excursions An excursion of general interest to an important section of the St. Lawrence Seaway Development is being arranged for the afternoon of Saturday, 13 July 1957, and a day's outing by steamer on the St. Lawrence River is being organized for Sunday, 14 July 1957. Three trips of mineralogical and geological interest will be available for the period following the Congress. Further details are given in the general information booklet. Exhibition It is planned to arrange an exhibition of apparatus and books of crystallographic interest at the Congress tteadquarters. Manufacturers and publishers wishing to receive detailed information should write to Dr A

A quantitative theory for the shapes of the absorption bands of F -centres is given on the basis of the Franck-Condon principle. Underlying the treatment are two simplifying assumptions: namely, ( a ) that the lattice can be approximately treated as a dielectric continuum; ( b ) that in obtaining the vibrational wave functions for the lattice, the effect of the F -centre can be considered as that of a static charge distribution. Under these assumptions, it is shown that the absorption constant as a function of frequency and temperature can be expressed in terms of the Bessel functions with imaginary arguments. The theoretical curves for the absorption constant compare very favourably with the experimental curves for all temperatures. Also considered in the paper are the probabilities of non-radiative transitions, which are important in connexion with the photo-conductivity observed following light absorption by F -centres. The treatment given differs from the qualitative considerations hitherto in one important aspect, namely, the strength of the coupling between the electron and the lattice is taken into account. The adiabatic wave functions for the F -centre electron required for the discussion are obtained by perturbation methods. The probability for an excited F -centre to return to its ground state by non-radiative transitions is shown to be negligible; similar transitions to the conduction band are, however, important if the excited state is separated from the conduction band by not much more than 0·1 eV. The temperature dependence of such transitions is complicated, but, for a wide range of temperatures, resembles e - w/k T . Tentative estimates show that the result is Consistent with the observed steep drop of the photo-conductive current with temperature.

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We study the effect of the induced interaction on the superfluid transition temperature of a Fermi gas with a Bose-Einstein condensation-Bardeen-Cooper-Schrieffer ͑BEC-BCS͒ crossover. The Gorkov-Melik-Barkhudarov theory about the induced interaction is extended from the BCS side to the entire crossover and the pairing fluctuation is treated in the approach by Nozières and Schmitt-Rink. At unitarity, the induced interaction reduces the transition temperature by about 20%. In the BCS limit, the transition temperature is reduced by a factor of about 2.22, as found by Gorkov and Melik-Barkhudarov. Our result shows that the effect of the induced interaction is important both on the BCS side and in the unitary region.

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