Prof. Encke on Hadley's SexlaJit. 



187 



one i = — 33'. If i had not been made = but equal to the 

 mean value of i', the deviation would be equal on both sides, 

 and this would perhaps be most convenient for the observa- 

 tion. If we designate however, in the present case, the place 

 of the image, if on the lower wire, by L, if in the middle be- 

 tween the lower and central cross wire, by ^ L, and if in the 

 same manner ^ U, and U denote similar positions with re- 

 gard to the upper wire, as also C with regard to the central 

 cross wire, — we may calculate the following table for the sex- 

 tant of this observatory. 



If the ring of the telescope be sufficiently steady, this table 

 of errors will be applicable for a long period, because I may be 

 considered as perfectly invariable, provided means are taken 

 to insure the parallel position of the mirrors. It is apparent 

 from this table how far from the middle the contact of the 

 images may be observed without committing considerable 

 errors ; and it is likewise clear that it is hardly to be expected 

 that any number of observations with a sextant, however great, 

 will give a large angle within three or four seconds ; partly on 

 account of the inferior power of the telescopes of the sextant, 

 and partly because all errors of observation with sextants with- 

 out a stand have always the same sign. It really appears, 

 that when this instrument first became known in Germany, its 

 powers were overrated. 



2B2 



XXVIII. An 



