S26 Cambridge Philosophical Society. 



correspond to rays of light. The principal theoretical deductions 

 are, — (1.) that the longitudinal vibrations of a ray are defined by a 



function of the form sin t:^ — Iz—a <^ /i+iA.\ X being the 

 breadth of the undulation, and a, e certain constants ; (2.) that light 

 from any source is in general composed of rays for which a and — 



are the same and X different ; (3.) that light coming immediately 

 from its origin is common light, whatever be the nature of the cause 

 producing it, and that to become polarised light, it must be acted 

 upon by reflexion, refraction, &c. ; (4.) that light coming imme- 

 diately from its origin is seen in all directions. 



Nov. 27, 1848. — Observations of the Aurora Borealis of Nov. 17, 

 1848, made at the Cambridge Observatory. By Professor Challis. 

 These observations relate principally to the corona, or point of 

 apparent convergence of the streamers, the remarkable display of 

 Nov. 17 being peculiarly favourable for noting the position of this 

 critical point. They were taken partly by estimation of distances 

 from stars, and partly by a small altitude and azimuth instrument 

 (called by the author a meteoroscope), which is furnished with a bar, 

 eighteen inches long, carrying at one end a rectangular piece whose 

 edges are horizontal and vertical, by looking at which through an 

 eyelet-hole, about the size of the pupil of the eye, at the other end, 

 the collimation is performed. Each observed position is compared 

 with the point of the heavens to which the south end of the dipping- 

 needle was directed at the time of observation. Tiiis point was 

 ascertained by means of observations of declinntion,- horizontal 

 force, and vertical force, taken at tlie Greenwich Observatory during 

 the prevalence of the aurora by Mr. Brooke's photographic pro- 

 cess, the results of which were communicated to the author by tlie 

 Astronomer Royal. It is assumed that the magnetic declination and 

 dip at Cambridge differ from those at Greenwich at any given time 

 by certain constant quantities, whether the magnet be disturbed or 

 not. These constant differences were derived from the following 

 formula : — 



V-Vo = 0-142518\ -I- 0-159548; 



D-Do= 0027713\ + 0-5135231, 

 in which V and D are the declination and dip at a place not very 

 distant from Greenwich, Vq and Do' the contemporaneous declina- 

 tion and dip at Greenwich, X the longitude of the place 7vest, in 

 seconds of time, and I the excess of its latitude in minutes above that 

 of Greenwich. These are merely formulae of interpolation by sim- 

 ple differences derived from the following data : — 



T ^ T tur i. Declination Dip 



Lat. Long. West. -^^i^^^,. in 1843. 



o / m. s. o / o I 



Greenwich 51 28-6 0-0 23 17-59 69 1-9 



Makerstoun 55 34-7 10 3-5 25 22-85 71 25-0 



Dublin '53 21-0 25 4-0 27 9-87 70 41*3 



