LA TRADIZIONE DEL MEZZO E LA NUOVA TEORIA DELLA LUCE DI LEONHARD EULER

Abstracttitle SUMMARY /title The object of this study is Leonhard Euler's physical optics as it is formulated in Nova theoria lucis et colorum (1746). The focus is on this particular work by Euler for two reasons: 1) Nova theoria represents undoubtedly the most comprehensive and systematic medium theory of the 18th century; 2) it contains the basic principles of Euler's conception of the nature of light, which he later mantained. The works of the most important advocates of this tradition (Huygens, Malebranche and Johann II Bernoulli) are here analysed, to give a historical frame to Euler's role in the medium tradition. Though these authors try elaborate a theory of light alternative to the emission theory, they never realize the contrast between the medium and the emission traditions. From this perspective, Nova theoria is a real transition point: Euler is fully aware of the antithesis between the two traditions; he compares them, he refutes the arguments in favour of emission theory and formulates an alternative one, that will substantially be the first and the most significant antagonist of emission model. The essay examines also the central questions of Euler's theory of light, i. e. how pulses are generated and propagated, the nature of the rays of light and the relations among pulse distance, frequency, and velocity.

Näherungslösungen für die Form kernmagnetischer Signale bei Impulsexperimenten in Festkörpern

NMR transients in solids have been calculated using the method of LOWE and NORBERG 1 and its extension by POWLES and STRANGE 2. Some remarks are given concerning the evaluation of the formal solution for the signal after several pulses.So far the influence of the pulse durations and of the longitudinal relaxation is neglected usually, higher terms of the power series expansion of the signals were discussed only with respect to corrections of their amplitudes. Some remarkable differences between these calculations and new experimental results to be reported here are explained — at least in principle — avoiding these limitations:The maximum amplitude of the echo after two 90°-pulses (phase difference of their modulations Δφ=90°) occurs not exactly at a time twice the pulse distance, the deviation is explained considering the duration of the pulses and the decay of the echo envelope.After two pulses with Δφ=0° an echo was found which should disappear only in an ideal twospin system. For a three pulse sequence the influence of the longitudinal relaxation is described; and in a multi-pulse sequence the calculated modulation of the amplitude of successive echoes was checked by experiments.