Prof. Encke on Hadley's Sexta7it. 183 



The detached telescope is now to be placed behind the sex- 

 tant, so that its line of collimation is as nearly as possible in 

 the same level with that of the telescope of the sextant, and is 

 to be directed to the same point. If the telescope has a large 

 apertuie, the intermediate position of the sextant will not be 

 any great obstacle to distinct sight. The sextant is now turned 

 180°, and its plane is again brought into a horizontal position; 

 if we now look into the detached telescope, the coincidence of 

 the two points of intersection of the cross wires of both tele- 

 scopes will prove that there is no error of inclination ; if they 

 do not coincide, the distance of the two points will be = 2 /'. 

 It will not be difficult to take one half of this distance, be- 

 cause, on the above supposition, we see through the detached 

 telescope, at the same time, the point previously determined, 

 and the cross wires of the telescope of the sextant. If the te- 

 lescope is therefore directed to the point of bisection of this 

 distance, and if the position of the telescope of the sextant is 

 then so corrected as to cover the intersection of the wires in 

 their new position, we shall have at least nearly i = 0, as will 

 be seen by repeating the operation by way of verification. The 

 ring into which the telescope of the sextant is screwed, is com- 

 monly furnished with means of correction by turning about 

 two points; otherwise the place in the field of vision is to be 

 marked by a new cross wire ; or, if the distance is small, taken 

 by estimation from its relative position to the lateral wires. 



In some trials in this observatory with a sextant made by 

 Troughton, in which at first very rough methods were applied, 

 and afterwards in applying more accurate ones the terrestrial 

 object was I'eplaced by the cross wires of an altitude circle, 

 a difference of 30" was found, the cause of which was entirely 

 to be attributed to the deficient methods of levelling first used. 

 With common care it will, however, be easy to ascertain i 

 as accurately as tlie power of the telescope of the sextant 

 will admit. By means of the sextant's telescope thus adjusted, 

 let the line of collimation of the detached telescope in a lateral 

 position be made horizontal, the sextant being placed hori- 

 zontally, and the position of the detached telescope being 

 changed until its cross wires cover that place of the field of 

 the sextant's telescope, for which i = 0. The angle which the 

 line of collimation in this new position makes with the former 

 object, may be measured by the sextant; let it = j). If 

 the sextant is now turned in the same horizontal plane until 

 the image of the former object in the same horizontal plane, 

 once reficctcd by the large mirror, is seen in the telescope, this 

 will furnish the means of delcrmiiiing /, If the cross wires 

 cover the object exactly in the various positions of the laigc 



mirror, 



