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THE chemical and physical properties which we think of as characteristic of transition metals, for example their variable valencies and the colours of their compounds, are almost all associated with the presence of an incomplete shell of d electrons. I n this Review we shall consider, qualitatively, two closely related theories which deal with the effect of the neighbours of a transition-metal ion on the behaviour of the d electrons of the metal, and hence on the properties of the ion. The first theory, usually called the crystal field or electrostatic theory, was developed by Bethe, Van Vleck, 2 and many others 39 4, t i in connection with studies on magnetism. The second theory, the molecular-orbital theory, was also discussed a t an early date by Van More recently each theory has been used to discuss the optical and chemical properties of a variety of compounds. We regard them both as expressing certain aspects of a more complete theory which we call ligand-field theory.

Concurrent rainbow trout (Oncorhynchus mykiss) stomach contents and invertebrate drift samples were collected during three 24-h periods in summer 1987. Feeding was discontinuous through the day on all dates. Mean stomach content weight was minimal after 0400 MST and sharply increased between 1000 and 1200 MST on all three dates. Feeding apparently did not occur after twilight. Mean stomach content weight was correlated with water temperature on two dates and was never correlated with invertebrate drift density for non-age-0 trout. Daily ration (wet weight) was 7% of live weight for non-age-0 trout and 21% for age-0 trout. Trichoptera, Ephemeroptera, and Diptera were most important in the diet; terrestrial insects and aquatic vertebrates were rare. The degree of selectivity varied through 24 h and the interpretation depended on the method of analysis used. Occurrence of low-drift cased Trichoptera larvae in stomachs was correlated with amount of filamentous algae ingested, indicating a degree of epibenthic foraging, although no diel pattern could be reliably discerned. Mean length of prey items in stomachs was larger that the drift in 83% of the samples. Our findings support experimentally derived decision rules-of-thumb for foraging trout: select larger prey items, select vulnerable prey, and relax selectivity when hungry.

Using Ray's results,2 the proof also generalizes immediately to the case where particles are killed at a rate V(x) throughout the region R as well as at the boundary. Only slightly more than continuity of V(x) almost everywhere in R is required. Similar results are expected to hold for the elastic-barrier case.