G-rubb— Registration of Star-Transits by Photography. 139 



They pass fastest at the Equator, and would be absolutely 

 stationary at the exact Pole if there happened to be a star there. 

 The clockwork arrangements, therefore, for carrying the plates 

 would have to be regulated according to the declination of the 

 star under observation — a difficult problem. 



I suggest the following as a possible convenient solution of the 



question : — 



The transit instrument itself would be of the ordinary form, 



Fig. 1. 



and mounted as usual on a pair of piers or columns, the only 

 difference necessary being that the object-glass, instead of being 

 as usual at the upper end of the tube, must be in the centre, where 

 the horizontal axis intersects the tube. 



The lower end of the tube would be quite open. The photo- 

 plate, p, would be carried, not on the tube, but on a metal arc 

 attached to the upper end of a polar axis, which is mounted in a 



