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

The semiclassical Franck-Condon principle is shown to be related to the more rigorous (``exact'') quantum-mechanical perturbation formula in the following ways: (1) the Franck-Condon formula can be derived from the ``exact'' formula by using a mean value approximation or by neglecting certain commutators; (2) if the electric dipole moments are treated as approximately independent of position, the Franck-Condon and the ``exact'' absorption (or emission) spectrum have the same zeroth, first, and second moments, i.e., the same integrated spectrum, mean absorption frequency, and breadth; (3) the errors in higher moments than the second become relatively unimportant at high temperatures. If the electron-nuclear interaction is sufficiently strong the errors are unimportant even at absolute zero. The use of a quasi-molecular description in a many particle problem is found to be possible only if the masses or stiffnesses are allowed to be temperature dependent. A detailed analysis is made of the case in which the energy difference between the two electronic states is a linear function of the vibrational coordinates—and the latter are describable by normal modes. ``Exact'' formulas for the absorption and emission spectrum are obtained.

No abstract