128 HYDROGRAPHIC SURVEYING 



of which is of transparent glass and the other half a mirror. The 

 movable arm, or index arm, BI, revolves around the point B. It 

 is fitted with a vernier at / and carries an index mirror B'C. 

 The rays of light from an object 5 reflect from the index mirror 

 to the mirror at A, and from this mirror to the telescope, 

 through which 5 can be seen. To measure the angle between 

 the lines of sight SB and HA , direct the telescope to H, which 

 can be seen through the transparent half of the horizon glass, 

 and revolve the index arm, by using the clamp M and tangent 

 screw T, until the reflected image of 5 coincides with H. When 

 in this position, the angle EBI equals one-half of the required 

 angle, but since the arc ED has each half degree marked as a 

 whole degree the angle can be read directly from the arc by 

 means of the vernier V. 



Adjustments of Sextant. There are four adjustments of the 

 sextant, as follows: 



1. To make the plane of the index glass perpendicular to 

 the plane of the limb. 



2. To make the plane of the horizon glass perpendicular to 

 the plane of the limb. 



3. To make the line of collimation of the telescope parallel 

 to the plane of the limb. 



4. To make the planes of the mirrors parallel when the 

 index reading is zero. 



First Adjustment. Place the index bar near the middle of 

 the limb; with the eye near the plane of the limb, observe 

 whether the limb as seen directly and its image as reflected 

 in the index glass form a smooth continuous curve; if they do, 

 the glass is perpendicular to the plane of the limb and the 

 adjustment is correct. But if the reflected limb appears to be 

 above that part of the limb seen directly, the glass leans for- 

 wards; if it appears to be below, it leans backwards. In 

 either case it is made perpendicular to the plane of the limb 

 by means of the adjusting screws at its base. 



Second Adjustment. Look through the telescope and horizon 

 glass toward a star or other well-defined distant object. Move 

 the index bar slowly until the reflected image passes over the 

 image seen directly. If these images coincide, the horizon 

 glass is perpendicular to the plane of the limb. If they do 



