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Since the time of Leverrier, it has been known that the ellipse

corresponding to the orbit of Mercury, after it has been

corrected for the influences mentioned above, is not stationary

with respect to the fixed stars, but that it rotates exceedingly

slowly in the plane of the orbit and in the sense of the orbital

motion. The value obtained for this rotary movement of the

orbital ellipse was 43 seconds of arc per century, an amount

ensured to be correct to within a few seconds of arc. This effect

can be explained by means of classical mechanics only on the

assumption of hypotheses which have little probability, and which

were devised solely for this purponse.