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Photochromism has been observed in single-crystal iron-doped rutile. Colouring occurs on irradiation with light of photon energy greater than 2.5 eV. The ratio of the activation energies for optical bleaching (2.5 eV) and thermal bleaching (0.5 eV) agrees with approximate theory. The colouring process is interpreted as the optical transfer of an electron from the valence band to an Fe3+ Square Operator centre where Square Operator signifies an iron-adjacent anion vacancy: the majority of the resultant holes are trapped by Fe3+ Square Operator ions on sites without an adjacent vacancy. Bleaching is a combination of optical and thermal release of electrons from Fe2+ Square Operator centres direct and thermal release of electrons from Fe2+ Square Operator centres into the conduction band followed by recombination with the trapped holes, and direct recombination between the Fe2+ Square Operator centres and non- trapped holes.

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There is a commonly held assumption that health interview surveys attempt to measure the same health status construct as clinical examinations. This assumption is most obviously manifested in validity studies which compare the results of health interview surveys with either clinical examinations or medical records. As a consequence they have found that the morbidity indicators used in the surveys are of minimal validity. This article argues that the assumption is mistaken. The concept of perceived morbidity is briefly examined as a step towards a solution to these difficulties and then a more comprehensive framework for the consideration of morbidity is proposed. The article concludes by briefly demonstrating the possibilities for analysis of the relationship between perceived morbidity and consequent behaviour arising from the application of the framework to data taken from the Australian Health Survey, 1977-78.