18 
Mints to travEll^r^ 
index at 60°; and, holding the sextant in the left hand, with the right 
move the index gently backwards and forwards, looking, as yon do so, 
obliquely into the index glass; then, if the image of the arc in the mirror 
appears in perfect continuation of the arc itself, the adjustment is perfect; 
when this is not the case, the index glass is out of adjustment. If the 
derangement is great, the sextant is for the time being useless; if small, 
it may possibly be remedied by means of certain screws sometimes 
fitted at the back of the glass; but it is better to leave it alone, as an 
inexperienced observer would most probably only make it worse. A 
man who has a thorough knowledge of his instrument can take off 
the frame, and get it put square and straight. A bad derangement may 
be remedied in this way; but it is, very evidently, a thing not to be 
rashly attempted. 
2nd Adjustment ,—Having screwed in the telescope, look through it and 
the horizon glass at the sun, or still better, a star, and move the index 
backwards and forwards, on each side of zero (0°), when the reflected 
image of the object ought to pass exactly over the object itself. If it does 
not do this, but passes either to the right or left of it, the horizon glass 
is out of adjustment, and its adjusting screw must be gently turned 
until the reflected image does pass directly over the object itself. 
3rd Adjustment .—Screw the telescope firmly into the collar, turn the 
eye-piece until two of the wires in the focus of the telescope are parallel to 
the plane of the instrument. Select two stars, not less distant from each 
other than 90°, bring them into exact contact at the wire nearest to the 
plane of the instrument; fix the index, and move the instrument so as to 
throw the images upon the upper wire; if the contact remains perfect 
the adjustment is perfect: if not, it must be rectified by the two opposing 
screws in the double collar, taking care to slacken one before tightening 
the other: the one to slacken is that on the side towards which the 
contact opens. 
Index Error .—When the index is set at zero (0°) on the arc, the 
horizon and index glasses should be parallel, and the two images of a 
distant object, as a star, should exactly coincide; when this is not the 
case, it may be remedied by turning a screw in the mounting of the 
horizon glass. If this adjustment is not made, there will be an error 
in the place of the beginning of the graduation; this is called the 
Index Error; its amount is easily determined, and, as it affects all angles 
