Page:Proceedings of the Royal Society of London Vol 1.djvu/190

Rh Dr. Herschel, having made these preliminary experiments, pro- ceeds to apply them to investigate the nature and magnitude of the star lately discovered by Mr. Harding. A regular series of obser- vations on this star are detailed, beginning on the 29th of September,

,and ending on the 11th of October. Of these we must necessarily conﬁne ourselves to mention merely the general result with the com clusions deduced by the author from the whole of the investigation. These conclusions are as follows:—

1. A ten-feet reﬂector will show the spurious or real discs of ce- lestial and terrestrial objects, when their diameter is one fourth of a second; and in favourable circumstances that diameter may be di- vided, by estimation, into two or three parts.

2. A disc of the above diameter, whether spurious or real, to be seen as a round well-deﬁned body, requires a magnifying power of 500 or 600, and must be sufﬁciently bright to bear that power.

3. A real disc of half a second in diameter will be so magniﬁed by the above-mentioned power, that it may be easily distinguished from a spurious one of equal size, the latter not being affected by magnifying power in the same proportion as the former.

4. The different properties of the inside and outside rays of a mirror, with regard to the appearance of a disc, will show Whether it is real or spurious, provided its diameter is more than one-fourth of a second. i

5. When discs, either spurious or real, are less thanone fourth of a second in diameter. they cannot be distinguished from each other, because the magnifying power is not sufﬁcient to make them appear round and well deﬁned.

6. The same kind of experiments are applicable to telescopes of diﬁ'erent sorts and sizes, but will give a different result for the quan- tity here stated at one fourth of a second. being more when the in- strument is less perfect, and less when it is more so.

The general results of Dr. Herschel’s observations on Mr. Harding's newly discovered celestial body, to which the name of Juno has been given, are,—

1. That it is in every respect similar to those discovered by lVIr. Piazzi and Dr. Olbers, so that Ceres, Pallas, and Juno, are cer- tainly three individuals of the same species.

2. That these bodies (the last of which appears to be the smallest,) are incomparany smaller than any of the planets; for a telescope_that will show a diameter of one fourth of a second, will not determine whether their discs are real or spurious, although a power of more than 600 has been applied to each of them.

3. That the criterion of the apertures of the mirror has, on account of the smallness of the object, been equally unsuccessful; every me- thod that has been tried only proving their resemblance to small stars.

4. That the deﬁnition of the term asteroid, formerly given by Dr. Herschel, will equally express the nature of Juno, which, an account of its similar situation between Mars and Jupiter, and its departure from the general condition of planets, by the smallness of its