Oj)j>osition of Geres. 457 



aspect was very nebulous, without a distinct disc, notwith- 

 standing the presence of bright moonlight, which, as he re- 

 marks, increases the sharpness of planetary definition, and 

 accordingly permitted Uranus to be seen with beautiful preci- 

 sion. The measures gave 3"'548 for Ceres ; for Uranus, which 

 till measured had appeared evidently the larger, 3""475. On 

 this occasion, the remote planet's disc appeared to exceed 

 that of the nearer one in sharpness as much as the 

 latter surpassed that of the planetary nebula near v 

 Aquarii [I Ijl. iv.]. February 12, bad definition, 3""621. 

 At this time our observers received a letter from Olbers, stating 

 that on the 10th he had estimated the disc of Uranus at least 

 twice as large as that of Ceres; which, in fact, might have 

 been expected from the limited aperture of his achromatic, only 

 3f inches. A comparison of the regular increase of measures 

 with the diminished distances of the planet from the earth 

 (the rate of which had not been known at the time of measure- 

 ment) proved that, assuming the disc to have been 2"'527 on 

 Jan. 26, it would have amounted to only 2" - 754 on Feb. 12, 

 and hence Schroter concluded that the greater proportional 

 increase must have been due to the increasing visibility of the 

 exterior strata of the atmosphere during the planet's approach 

 to the earth; as had been, he considered, the case with the 

 comet of 1799. Feb. 18, notwithstanding a full moon, the 

 measure amounted to 3"' 048, with a cometary aspect. Mar. 5, 

 clear evening, but no disc perceptible. Mar. 6, more distinct, 

 and though very cometary, measuring 3" "843, whence he in- 

 ferred a variable state of its own atmosphere. Uranus ap- 

 peared at the same time perfectly defined, and like a miniature 

 Jupiter without the polar flattening. Mar. 7, bad definition; 

 3" - 965. Mar. 8, 3"*994, the brighter centre reduced by pass- 

 ing clouds to a much more defined disc of 3"*037. Mar. 14, 

 disc more defined in strong moonlight, 3"' 9 79, or in sharper 

 glimpses, 3"'342. His most excellent 10-ft. achromatic, by 

 P. Dollond, of 3^ inches aperture, never showed any mea- 

 surable disc or distinct coma; just as it was unable to reach, 

 in the great nebula of Orion, many details of faint luminosity 

 which the 13-ft. reflector showed with perfect distinctness. 

 Opposition, Mar. 16. Mar. 20, disc more defined than re- 

 cently, and brighter ; somewhat exceeding even Uranus : 

 4 ,/ -105, and for the densest portion 3"'263. Mar. 28, 4"-021, 

 and through flying clouds 2 //- 915 ; keeping the same proportion 

 as before. 



Here is certainly a strong case, both as regards positive 

 nebulosity and actual magnitude, and one which would have 

 appeared still stronger in the latter respect, if we had recounted 

 all the precautions taken by Schroter and Harding to ensure 



