adjustments Of THe sextant. 19 
alike, it is usual to admit the existence of this source of error, and apply 
correction for it, in preference to making the adjustment. 
To find the Index Error by a Star .—Set the index at zero (0°), screw in 
the telescope, and, with the tangent screw, make the two images of a 
star, as seen through the telescope, coincide; then the reading on the arc 
will he the index error. Subtractive when the reading is to the left of 
zero, additive when to the right. 
By the Sun .—Clamp the index at about 80' to the left of zero, and 
looking through the telescope at the sun, the images will be seen nearly 
in contact; make this contact perfect with the tangent screw, take the 
reading, and call this “ on the arc ”; next, set the index, at about 30' to 
the right of zero, and make the contact of the two images perfect as 
before, take the reading, and call it “off the arc”: half the difference 
of these two readings is the Index Error. 
Examples. 
(0 
t 
n 
0 ) 
/ 
// 
On the arc 
33 
10 
On the arc 
29 
3° 
Off the arc.. 
29 
30 
Off the arc 
33 
10 
2) 3 
40 
2 ) 3 
40 
Index coir, subtract 
= 1 SO 
Index corr. add 
= 1 50 
As a check on this observation, for inexperienced observers, it may be 
hoted that one-fourth of the sum of the readings on and off the arc 
ought to be the sun’s semi-diameter, as given in the ‘ Nautical Almanac.’ 
Centering Error .—In addition to the foregoing, every sextant is 
liable to errors caused by * 
1. The centre of the pivot of the index-bar carrying the vernier 
not being identical with the centre of the arc. 
2. Imperfect graduation of the arc. 
3. Flexure of the whole instrument caused by irregular expansion 
under the heat of the sun. 
4. Shocks or blows which may cause bending of parts of the frame, 
or of the index bar, and thus cause eccentricity between the 
vernier and arc. t 
c 2 
