458 Opposition of Ceres. 



correct results, as well as the instances of high accuracy pre- 

 viously attained in the use of the same instrument — the pro- 

 jection-micrometer. This very simple apparatus, consisting 

 only of a number of suitably illuminated discs, differing in size 

 and variable in distance, among which the telescopic image is 

 projected by the simultaneous use of both eyes, seems worthy 

 of more attention than it has received, especially at the hands 

 of the possessors of Newtonian reflectors, for which it is best 

 adapted. It has much advantage m the convenience of illu- 

 mination, and still more in not being sensibly affected by 

 slight inaccuracies of workmanship or adjustment ; but it 

 requires, probably, a good deal of practice to secure ultimate 

 correctness. But while Schroter and Harding had good reason 

 to be satisfied with their own proceedings, a strange dis- 

 crepancy in the observations on this side the water must have 

 proved not a little annoying to them, as it certainly was 

 startling to the astronomical world. After a very fair agree- 

 ment in this respect for twenty years between the observatories 

 of Slough and Lilienthal, Sir W. Herschel, employing the 

 same mode of measurement, had been deducing results of 

 marvellous contrariety with regard to the two new planets, 

 or, as he preferred to call them, asteroids, Ceres and Pallas ; 

 allowing to the former a diameter of only //- 351 at its mean 

 distance, equal to about -^ of the measure obtained at Lilien- 

 thal ! "Certainly, - " as Schr. remarks, " never has a contrast 

 altogether so extraordinary existed, since practical astronomy 

 has admitted of accurate and minute measurements." The 

 Hanoverian observer set himself, therefore, with all dili- 

 gence, to examine the possible cause, and satisfied himself of 

 the correctness of his own results by arguments which, as 

 usual, he has worked out with painstaking copiousness of 

 detail, but of which we can only give an outline. He remarks 

 — and, it must be owned, with apparent justice — that a disc of 

 such minute dimensions, shining with reflected and hazy light, 

 could not by any possibility have been circumstantially studied 

 by each observer with the greatest agreement as to physical 

 aspect : that his own repeated comparisons with Uranus con- 

 firm the apparent size he assigned to Ceres : that Harding saw 

 it as large again as the 1st satellite of Jupiter, instead of six 

 times smaller, according to lj[ ; s measure : and that on such a 

 supposition it must have been perfectly imperceptible in the 

 finder at Lilienthal, which, on the other hand, showed it twice 

 as large as that satellite. An error, therefore, he believes, 

 must have crept into Ill's measurements, and he proceeds to 

 trace it. He points out the incorrectness of drawing con- 

 clusions from only the smallest (not from a mean) of three such 

 very discordant values, 0"'40, 0"*38, and 0"-22, and found by 



