centre, where they are sufficiently condensed to produce the appear-
ance of a nucleus. These we are told are the most magnificent objects that can be seen in the heavens.
7. Nebule—These, it is thought, may be resolved into the three last—mentioned species, only removed to such a distance that they can only be seen by means of the most powerfid telescopes.
8. Stars with burrs, stellar Nebula—These are thought to be clusters of stars, at great distances. the light of which is gathered so nearly into one point, as to leave but just enough of it visible to pro- ‘duce the appearance of burrs.
9. Milky nebulosz'ties.——These phaenomena are probably of two different kinds, one of them being deceptions; namely, such as arise from extensive regions of closely connected clustering stars contigu- ous to each other, like those that compose our milky way: the other, on the contrary, being real, and possibly at no very great distanc'e from us. The milky nebulosity of Orion, discovered by Huygens is given as an instance of this singular appearance.
10. Nebuloas storm—Whether these he the effect of the atmo- spheres of certain stars remains yet to be determined; and indeed every thing respecting the nature of these appearances is still in- volved in much doubt and obscurity.
ll. Planetary Nebula ,- and 12. Planetary Nebula with centres.— These also, though objects manifestly distinct from _the former ones, are as yet so imperfectly known, as to baffle allreasoning concealing their nature and habits; and Dr. Herschel contents himself for the present with merely inserting the few he has observed in his catal- logue.
Here follows the copious catalogue of Nebula, &c., which being a continuation of two preceding papers of the like nature, and arranged in the same manner, requires no further explanation.
In a communication on this subject, published in the volume of the Philosophical Transactions for the year 1800, Dr. Wollaston ac- counted for various singular phaenomena of horizontal refraction by certain gradual changes in the density of the refracting medium. Hav- ing since perused what M. Monge has published in the Mémoz'res sur l'Egypte, concerning the appearance known to the French by the name of Mirage, where it is ascribed to permanent rarefied strata of air near the surface of the earth; our author, having reconsidered the subject, and finding that the facts related by the French philo- sopher accord entirely with his own theory, declares here that he still adheres to his former opinion, and assigns his reasons for not de- parting from it.
The chief of these reasons is, that the definite reflecting surface,
