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A study of early and later records of observations disclosed to him, in 1851, an apparent progressive approach of the inner edge of the bright ring to the planet. The rate of approach he estimated at about fifty-seven English miles a year, or 11,000 miles during the 194 years elapsed since the time of Huygens. Were it to continue, a collapse of the system must be far advanced within three centuries. But was the change real or illusory a plausible, but deceptive inference from insecure data? M. Struve resolved to put it to the test. A set of elaborately careful micrometrical measures of the dimensions of Saturn's rings, executed by himself at Pulkowa in the autumn of 1851, was provided as a standard of future comparison; and he was enabled to renew them, under closely similar circumstances, in 1882. But the expected diminution of the space between Saturn's globe and his rings had not taken place. A slight extension in the width of the system, both outward and inward, was indeed, hinted at; and it is worth notice that just such a separation of the rings was indicated by Clerk Maxwell's theory, so that there is an
A learned discussion by Dr. T. J. J. See, moreover, enforces the belief that Sirius was absolutely red eighteen hundred years ago. Roy. Trans., vol. cxci. Astr. Pac. Roy. Roy. Trans., vol. cliv., p. 413. Roy. Roy. Roy. Trans., vol. cxci. Roy. Jour., vol. xi., p. 262; Proc. Roy. Amer. Roy. Jour. of Sc., vol. xxxix., p. 46; Vogel, Astr. Nach. Micrometrical measures are always thus executed.
A series of measures from sixty-two similar images gave the distance and position-angle of its companion with about the same accuracy attainable by ordinary micrometrical operations; and the method and upshot of these novel experiments were described in three papers remarkably forecasting the purposes to be served by stellar photography. The sacrifice was recompensed by conspicuous success.
The compression at once caught Barnard's trained eye in 1894, when he undertook at Lick a micrometrical investigation of the system; and he was surprised to perceive that the major axis of the elliptical surface made an angle of about 28° with the line of travel pursued by the satellites.
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