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D'Arrest, indeed, protested against it, and Ångström, in 1868, substituted atmospheric quality for mere colour as a criterion of age and temperature. His lead was followed by Lockyer in 1873, and by Vogel in 1874.
The inference was, then, an easy one, that the meteors were pursuing the same path with the comet; and it was separately arrived at, early in 1867, by Weiss, D'Arrest, and Galle. But Biela travels in the opposite direction to Tempel's comet and its attendant "Leonids"; its motion is direct, or from west to east, while theirs is retrograde.
And as having the same meanings let me not omit the remarkable fact concerning the planetoids discovered by D'Arrest, that "if their orbits are figured under the form of material rings, these rings will be found so entangled, that it would be possible, by means of one among them taken at hazard, to lift up all the rest," a fact incongruous with Laplace's hypothesis, which implies an approximate concentricity, but quite congruous with the hypothesis of an exploded planet.
Chacornac observed it at Marseilles in 1854, but was confounded four years later to find it vanished. D'Arrest missed it October 3, and redetected it December 29, 1861. It was easily seen in 1865-66, but invisible in the most powerful instruments from 1877 to 1880.
Moreover, a fairly bright adjacent nebula, perceived by O. Struve in 1868, and observed shortly afterwards by d'Arrest, has totally vanished, and was most likely only a temporary apparition. These are the most authentic instances of nebular variability.
Displacements which would be conspicuous in stars might easily escape detection in ill-defined, hazy masses. Thus the measures executed by d'Arrest in 1857 have not yet proved effective for their designed purpose of contributing to the future detection of proper motions. Some determinations made by Mr. Burnham with the Lick refractor in 1891, will ultimately afford a more critical test.
D'Arrest remarked in 1851, when only thirteen minor planets were known, that supposing their paths to be represented by solid hoops, not one of the thirteen could be lifted from its place without bringing the others with it. The complexity of interwoven tracks thus illustrated has grown almost in the numerical proportion of discovery.
The same school-friend sometimes took me to the Observatory, to see old Professor d'Arrest a refined and sapient man and there, for the first time, I saw the stellar heavens through a telescope. I had learnt astronomy at school, but had lacked talent to attain any real insight into the subject.
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