GAUSS AND WEBER ON TERRESTRIAL MAGNETISM. 51 



at Gottingen in the first observations ; but has completely ceased 

 since each lamp has been provided with a copper chimney, di- 

 rected to the side. 



As in the term-observations several observers are required, 

 there may be a considerable difference in the distance at which 

 distinct vision is obtained by the several individuals. If a short- 

 sighted person comes to the telescope adjusted for a long- 

 sighted person, some alteration will be required for distinct 

 vision. The use of a concave glass would be inconvenient and un- 

 advisable, on account of the considei'able loss of light. The mere 

 sliding of the eye-tube is not sufficient, as, although the image 

 of the scale might thereby be rendered distinct, the cross threads 

 would remain indistinct, and would have a parallax in respect 

 to the image of the object. It would be necessaiy, therefore, 

 (with the construction which the telescopes employed in these 

 observations usually have) that the cell containing the cross 

 threads should be moveable in the eye-tube, and that it should 

 be brought nearer to the lens in the eye-piece ; but this requires 

 a practised hand, takes time, and for other reasons is not to be 

 recommended for the present case. The difficulty may, however, 

 be got over in a very simple manner, if the following plan be 

 adopted. The eye-tube in the telescope, and the cross threads 

 in the same, are to be so adjusted previous to the observations, 

 that the most short-sighted among the observers can see perfectly 

 distinct both the image of the scale and the cross threads ; when 

 a longer-sighted person arises in turn, he has merely, without 

 , displacing the eye-tube or the cross threads, to draw out the 

 glass nearest the eye so far that he can define perfectly well 

 the cross threads, and with this a completely distinct vision of 

 the image of the scale is necessarily connected. A short-sighted 

 ' person coming in turn has merely to make an adjustment in the 

 contrary way. 



For the purpose of proving the undisturbed state of the tele- 

 scope, a mark is employed, which is placed at such a distance 

 that it may be seen distinctly with the same position of the 

 eye-piece as is required for the distinct vision of the image of 

 the scale ; this consists, in the Gottingen obserA^atory, of a small 

 vei'tical line on the northern wall*. Previously to the commence- 



• With respect to this arrangement, I may here observe that a mark for the 

 verification alhided to must be considered as indispensable. Previously to 

 the IniiUlintr of the present Gottingen magnetic observatory, it was doubted 



D 2 



