HYDROGRAPHIC SURVEYING 129 



not coincide, the horizon glass is adjusted by an adjusting 

 screw placed under, behind, or beside the glass, according to 

 the construction of the instrument. 



Third Adjustment. Place the sextant in a horizontal posi- 

 tion on a table or other support, and direct the telescope 

 at some well-defined point or mark about 20 ft. away. Place 

 two small blocks of equal height on the limb, one near each 

 extremity. These blocks should be of exactly equal height, 

 so that a line of sight over their tops will be parallel to the 

 plane of the limb, and should be at the same height above the 

 limb as the center of the telescope. Sight over the tops of 

 the two blocks in the direction of the point or mark sighted 

 through the telescope, and note whether the line of sight 

 intersects the mark. If it does not, but falls above or below 

 the mark, the telescope is not parallel to the limb. It can be 

 made parallel to the limb by means of the screws in the collar 

 that holds the telescope. This adjustment, however, is not 

 usually made unless the error is considerable, because a slight 

 lack of parallelism between the line of sight and the plane of 

 the limb does not appreciably affect the angular measurements 

 on the limb. 



Fourth Adjustment. Set the index at zero, look through the 

 telescope toward a star and note whether the direct and reflected 

 images of the star coincide. If they do, the adjustment is 

 correct. If they do not, move the index bar until they do 

 coincide, and clamp it in this position. The reading of the 

 index when in this position is called the index error. This 

 error can be corrected by means of screws at the back of the 

 index glass, which cause it to revolve about an axis perpen- 

 dicular to the plane of the limb. To make the correction, set 

 the index bar at zero and, by turning the screws, revolve 

 the index glass until the two images exactly coincide. This 

 adjustment will usually disturb the previous adjustment of 

 the index glass, and, as a rule, it is not made unless the index 

 error is greater than 3 min. 



When the index error is less than 3 min., it is usually applied 

 as a correction to all observations. If the error is off the arc, 

 that is, if the index is to the right of the zero mark, it is addi- 

 tive, or plus, and should be added to all readings. If the 



