Page:Philosophical Transactions - Volume 095.djvu/74

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Dr. Herschel's Experiments on the Means

requires a distinct magnifying power of 5 or 6 hundred, and must be sufficiently bright to bear that power.

(3.) A real disk of half a second in diameter will become so much larger by the application of a magnifying power of 5 or 6 hundred, that it will be easily distinguished from an equal spurious one, the latter not being affected by power in the same proportion as the former.

(4.) The different effects of the inside and outside rays of a mirror, with regard to the appearance of a disk, are a criterion that will show whether it is real or spurious, provided its diameter is more than 1/4 of a second.

(5.) When disks, either spurious or real, are less than 1/4 of a second in diameter, they cannot be distinguished from each other; because the magnifying power will not be sufficient to make them appear round and well defined.

(6.) The same kind of experiments are applicable to telescopes of different sorts and sizes, but will give a different result for the quantity which has been stated at 1/4 of a second of a degree. This will be more when the instrument is less perfect, and less when it is more so. It will also differ even with the same instrument, according to the clearness of the air, the. condition, and adjustment of the mirrors, and the practical habits of the observer.

With regard to Mr. Harding's new starry celestial body, we have shown, by observation, that it resembles, in every respect, the two other lately discovered ones of Mr. Piazzi and Dr. Olbers; so that Ceres, Pallas, and Juno, are certainly three individuals of the same species.