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Published online by Cambridge University Press: 25 April 2016
The origin of QSO redshifts is an important problem for which there is still no definite answer (Burbidge 1979). Recent observations by Walsh et al. (1979) of what is probably a gravitational lens associated with the twin QSO’s 0957+ 561 A,B offers the possibility of testing whether the redshift of the QSO involved is cosmological in origin. In general, if a gravitational lens is responsible for splitting the image of a QSO into two or more distinct components, it is possible to calculate the redshift of the QSO provided we have enough observational data involving the lens. The calculated redshift must then be consistent with the observed redshift if it is entirely cosmological in origin. The quantities that must be observed before the redshift can be calculated are the differences in the light travel times along the different paths from the QSO to us, the redshift of the lens, the angular separation of the images, and their relative intensities.