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It is an open question whether or not this effect exists, and at

the present time (1920) astronomers are working with great zeal

towards the solution. Owing to the smallness of the effect in the

case of the sun, it is difficult to form an opinion as to its

existence. Whereas Grebe and Bachem (Bonn), as a result of their

own measurements and those of Evershed and Schwarzschild on the

cyanogen bands, have placed the existence of the effect almost

beyond doubt, while other investigators, particularly St. John,

have been led to the opposite opinion in consequence of their

measurements.