Royal Astronomical Society. 579 



which he adopts of luting together the crown and flint-glasses form- 

 ing the achromatic combinations, by means of balsam of Canada 

 almost cold, this process being not subject to the inconvenience at- 

 tached to the use of mastic in drops recommended by other artists. 

 He also suppresses the use of diaphragms in the tubes of his tele- 

 scopes, which he lines with black velvet. 



The author's micrometrical lunette consists of a telescope bent at 

 right angles, in the manner of a Newtonian reflector, carrying a 

 sight on the object-end and a small perforated mirror on the eye- 

 piece, which combination permits the observer to see at once, and 

 with the same eye, an object at a fixed distance, and the image of the 

 object produced by the telescope. All the usual micrometrical and 

 illuminating methods are applicable to the sight, on which may be 

 traced divisions as small as we choose, being read by a small tele- 

 scope parallel to the first, and placed before the metallic mirror. 

 By the author's arrangement, the errors to which common micro- 

 meters are subject, depending on the thickness of the wires, the im- 

 perfection of the screws, &c. are got rid of. He considers his mi- 

 crometer as particularly applicable to geodetic operations, and suit- 

 able for the purposes of the surveyor as well as of the astronomer. 



Observations of the Aurora Borealis. By Robert Snow, Esq. 



In this paper Mr. Snow records his observations of this interest- 

 ing phenomenon made at Ashurst and Dulwich, from the autumn 

 of the year 1834 to the autumn of 1839, within which period several 

 remarkable aurora? appeared ; among which, as particularly de- 

 serving notice, he describes the various appearances of those of No- 

 vember 17, 1835, and of February 18, 1837 ; the latter being ren- 

 dered additionally remarkable by its happening on the same evening 

 as the occultation of Mars by the moon. Other fine aurora de- 

 scribed by the author appeared on November 12, 1837 ; on Sep- 

 tember 13, 1838; and on January 19 and September 3, 1839. 



The author deduces from his observations the following invariable 

 circumstances of the phenomenon : — That the aurora may be ex- 

 pected at any season of the year ; that it assumes nearly every va- 

 riety of colour ; that it resembles both in shape and motion every 

 variety of ordinary cloud ; that its appearances are, in the course of 

 the same evening and without any determinate order, undulating, 

 radiating, and streaming, with other capricious forms not easily ex- 

 pressible ; that the length of time during which it is visible is very 

 uncertain ; that it appears to the eye (geometrical considerations 

 apart) as if it existed at various distances from the earth's surface ; 

 that, although for the most part it is not influenced by the presence 

 of clouds, it occasionally tinges them with its own prevailing colours ; 

 that this has been noticed only when the clouds are low ; that there 

 are also certain lofty cirrous clouds, which have the appearance of 

 arranging themselves in peculiar bands or strata, as if in connexion 

 with the aurora ; that these strata are visible during daylight, when 

 the visibility of the dark portion of the arch has sometimes been 

 strongly suspected ; that the stars are seen both well and ill defined 

 through the auroral light and the auroral darkness ; that it is by no 



