THE WANDERINGS OF TPIE NOETn POLE.* 



By Sir Robert Ball, F. R. S. 



On a recent visit to Cambridge, Prof, Barnard, the discoverer of tlie 

 liftli .satellite of Jupiter, exhibited at the Cavendish Laboratory his most 

 interesting- collection of pliotographs ma<le at the Lick Observatory. 

 These pictures were obtained by a 6-inch photographic lens of 3 feet 

 focus, attached to an ordinary equatorial, the telescope of which was 

 used as a guider when it was desired to obtain a picture of the stars 

 with a long exposure. Among the advantages of this jirocess may be 

 reckoned the large field that is thereby obtained, many of the plates that 

 he exhibited being as mucli as 4 degrees on the edge. I am however 

 not now going to speak of Barnard's marvellous views of the milky way, 

 nor of the plate on which a comet was discovered, nor of the vicissitudes 

 of Holmes's comet, nor of that wonderful picture in which Swift's comet 

 actually appears to l)e producing, by a process of gemmation, an off- 

 shoot which is evidently adapted for an independent cojuetary exist- 

 ence. The picture to which I wish specially to refer in connection with 

 our immediate subject is one in which the instrument was directed 

 towards the celestial pole. In this particular case the clock-work which 

 IS ordinarily employed to keep the stars acting at the same point of the 

 plate was dispensed with. The telescope, in fact, remained iixed while 

 the heavens rotated iii obedience to the diuriuil motion. Under these 

 circumstances each star, as minute after minute passed by, i)roduced 

 an image on a different part of the platerthe consequence of which was 

 that the record which the star was found to have left, when the picture 

 was developed, was that of a long trail instead of a sharply defined 

 point. As each star appears to describe a circle in the sky around the 

 pole, and as, in the vicinity of the pole, these circles were small enough 

 to be included in the plate, this polar photograph exhibits a striking- 

 spectacle. It displayed a large niunber of concentric circles, or rather, 

 I should say, of portions of circles, for the exposures having lasted for 

 about four hours, about one- sixth of each circumference was completed 

 during that time. 



*From the Fortnightly Review, August, 1893 ; vol. liv, pp. 171-183. 



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