100 



centre, where they are sufficiently condensed to produce the appear- 

 ance of a nucleus. These we are told are the most magnificent ob- 

 jects that can be seen in the heavens. 



7. Nebula. 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 powerful 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 nebulosities. These phenomena 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 distance 

 from us. The milky nebulosity of Orion, discovered by Huygens, is 

 given as an instance of this singular appearance. 



10. Nebulous stars. Whether these be the effect of the atmo- 

 spheres of certain stars remains yet to be determined ; and indeed 

 every tiling respecting the nature of these appearances is still in- 

 volved in much doubt and obscurity. 



11. 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 all reasoning concerning 

 their nature and habits ; and Dr. Herschel contents himself for the 

 present with merely inserting the few he has observed in his cata- 

 logue. 



Here follows the copious catalogue of Nebulae, &c., which being a 

 continuation of two preceding papers of the like nature, and arranged 

 in the same manner, requires no further explanation. 



The Bakerian Lecture. Observations on the Quantity of horizontal 

 Refraction ; with a Method of measuring the Dip at Sea. By Wil- 

 liam Hyde Wollaston, M.D.F.R.S. Read November 11, 1802. 

 {Phil. Trans. 1803, p. 1.] 



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 phenomena 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 Memoir es sur 

 VEgypte, 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, 



