Opposition of Geres. 459 



actual experiment that measures by the projection-micrometer 

 could not be depended upon beyond the distance at which a good 

 sight can read ordinary writing, such as from five to seven 

 feet, or somewhat more : within these limits, measures at the 

 utmost will not differ more than -^ ; but at great distances, to 

 say nothing of the attendant inconveniences, the comparison- 

 discs appear, from irradiation, of an entirely deceptive magni- 

 tude. On looking at such a disc, placed at a distance of 173 

 feet, with both eyes, the left free, the right directed through 

 a small sextant- tube without lenses, he found the image in the 

 left eye five times larger than in the right : and the reversal of 

 the appearance, on changing the tube to the other eye, proved 

 the certainty of the illusion, which diminished with the dis- 

 tance, but did not disappear till the disc was only eight feet 

 from the eye. And hence, as ^ had compared Ceres with a 

 disc at nearly 162 feet, the extraordinary discrepancy was 

 accounted for ; a conclusion which was confirmed by the cir- 

 cumstance, that the deviation in IjPs measure of Pallas, with a 

 disc placed as far as 1 78 feet, had been even greater than in the 

 case of Ceres. The first suspicion of this cause of error, he 

 says, in the recollection of his own innumerable measurements 

 with the projection-micrometer, " made a frightful impression 

 upon my whole soul •" from which, however, he was entirely 

 relieved by finding that the dissimilarity wholly vanished 

 considerably beyond the Hmits of such distances as he had 

 invariably employed for twenty years. Ijl himself had, in 

 measuring Uranus, discovered that equal comparison-discs at 

 equal distances from the eye would appear of unequal size if 

 unequally illuminated ; and that his disc-micrometer gave the 

 measure ^- too small from irradiation : as indeed must natu- 

 rally have resulted from his distances of 49 and 57 feet. 

 And Schr. fully confirmed his own view, by measuring sub- 

 sequently the little planet Juno with a disc at 1434- feet; 

 the result being //- 50 instead of 2 //- 526 as given at a due 

 distance. In addition to all this, he also urges the unsuit- 

 ableness of employing magnifying powers so high as to 

 lose the rarer parts of the nebulosity, and reduce the visible 

 object to merely its brighter centre : and he remarks, that 

 had the diameter of Ceres been so extremely small, its 

 planetary light never would have been distinctly seen, 

 as it was on Dec. 19, 1804, at a much greater distance from 

 the earth (2*673 instead of I. - 9 to 1*6), with an achromatic of 

 scarcely two inches aperture, magnifying 21 times; nor would 

 it, he maintains, in the 13-ft. reflector, have preserved its 

 disc of about l" - 5 through a green glass which greatly reduced 

 the magnitude of a brighter star ; nor could it have been so 

 much more conspicuous than Titan, to which his measures gave 



