4516 HYDROGKAPHIC MANUAL PaGE 366 



No adjustment is provided for the binocular telescope now attached to sextants 

 used by the Coast and Geodetic Survey (see {d) above). The telescope is installed 

 by the manufacturer or by the Instrument Division of the Bureau so that its line of 

 sight is parallel to the arc. Older navigating sextants, to which binocular telescopes 

 have not been affixed, may be provided with an adjustment by which the line of colli- 

 mation can be corrected. The method to be used is described on page 123 of Bowditch 

 or in any good treatise on the use of the sextant. 



Chv^ing to the construction of the sextant there is a small error in small angles, due 

 to the fact that the vertex of the measured angle is actually back of the eye. It is 

 important only when distances are to be derived from small sextant angles, as in the sex- 

 tometer method, or perhaps in small vertical or depression angles. The efi'ect and 

 amount of this error are considered in 2382. 



4516. Sextant Adjustments 



The horizon and index mirrors must be adjusted by the movements provided to 

 the holding frames to eliminate errors (e), (/), and (g) in 4515. 



Referring to figure 76, the index mirror A must first be made perpendicular to the 

 plane of the instrument. To make this adjustment set the index arm near the middle 

 of the graduated arc and hold the sextant with the eye close to the index mirror and as 

 nearly as practicable in the plane of the sextant, with the graduated arc away from the 

 eye. Observe the graduated arc direct and its reflection in the index mirror, moving 

 the arc slowly back and forth, and see whether the arc and its reflection appear to form 

 an apparently continuous unbroken arc. If so, the mirror is perpendicular to the plane 

 of the arc; otherwise, correct its position by adjusting the screw at the back of the 

 frame until it is perpendicular. 



The horizon mirror B should now be adjusted perpendicular to the plane of the 

 sextant. With the index mirror in adjustment, set the index arm near zero and hold 

 the sextant so its plane is vertical and sight at the sea horizon. Bring the horizon as 

 seen direct and its image in the horizon muTor into coincidence by moving the tangent 

 screw. Incline the sextant so that its plane makes a slight angle with the horizon and 

 see if the horizon and its image are still in coincidence. If not, adjust the position of the 

 horizon mirror by means of the screws attached at the back of the frame until coinci- 

 dence is obtained. 



This adjustment of the horizon mirror may be made at night by pointing the sextant 

 at a star and moving the index arm so that the reflected image of the star appears to 

 pass the star as seen direct. If they are not in coincidence in passing, the horizon glass 

 needs to be adjusted as described above until one image passes directly in front of the 

 other when tested. 



The index error (g) in 4515 and its adjustment are considered in 4517. 



4517. Index Error 



The index error of a sextant is due to the fact that the reflecting surfaces of the 

 two mirrors are not parallel with each other when the index arm is set at zero. The 

 adjustments described in 4516 having been previously made, set the index arm exactly 

 at zero and point the sextant at some distant object, usually the sea horizon, and 

 rotate the horizon mirror by means of the adjusting screw (A^ in fig. 77) to one side of 

 its frame until the direct and reflected images are in coincidence. This adjustment 



