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frame at t = 0, i.e. with its origin at (r0, ~00, z0) in our basic frame and with its x axis perpendicular to our basic z axis and pointing directly away from it. It can be shown that we should put It should be remembered that (rs, ~, z~) should refer to a left-handed helix in a right-handed frame.I should like to thank Mr G. Kreisel for a number of interesting discussions and in particular for suggesting the use of Parseval's theorem at a crucial point.