3. EQUIPMENT AND INSTRUMENTS 



47 



down to coincidence and up to coincidence 

 alternately, taking a mean of the results. If 

 the zero of the vernier is to the right of the 

 zero of the arc, or off the arc, the correction 

 is positive and should be added to measured 

 angles. If it is to the left, or on the arc, the 

 correction is negative and should be sub- 

 tracted from the measured angles. 



(b) Substitute a star at night for the sea 

 horizon, point the sextant at the star and 

 bring the direct and reflected images into 

 coincidence and read the setting of the index 

 arm. Repeat several times and use a mean 

 of the results as above. 



(c) Measure the apparent diameter of the 

 sun with the sextant held vertically, bring- 

 ing the upper limb of the reflected image 

 to touch the lower limb of the direct image. 

 Read the angle. Then bring the lower limb 

 of the reflected image to touch the upper 

 limb of the direct image and read the setting 

 off the arc. Half of the difference of the 

 two readings is the index correction, positive 

 or negative as the larger of the two values is 

 off or on the arc, respectively. For example, 

 if the diameter measures 33'50" on the arc 

 and 32'40" off the arc, the index correction 

 would be 1/2 (33'50"-32'40")=35" (Minus). 

 Several such observations should be taken 

 and the mean used. The accuracy of the re- 

 sult may be verified by comparing the sun's 

 semidiameter for the date of observation as 

 taken from the Nautical Almanac with one- 

 quarter of the sum of the two readings irre- 

 spective of sign. 



3-12 Sextant mirrors. — Stellite mirrors 

 are used almost exclusively in the Coast 

 and Geodetic Survey. Stellite is a trade name 

 for an alloy of chromium, cobalt, and tung- 

 sten. These mirrors have a front reflecting 

 surface which eliminates errors due to non- 

 parallelism of the two surfaces of glass mir- 

 rors. The surface must be polished, op- 

 tically flat, free of scratches and other 

 inperfections. 



Stellite mirrors are practically indestruc- 

 tible if given reasonable care. The material 

 is expensive and difficult to obtain. They 

 should not be expended when no longer con- 

 sidered usable in the field. Unserviceable 



mirrors should be returned to the Washing- 

 ton OflRce for repolishing and checking for 

 optical flatness. 



3-13 Use of sextant. — To measure an 

 angle between two objects with a sextant, 

 the observer looks at the one on the left over 

 the top of the horizon mirror and moves the 

 index arm until the reflection of the right- 

 hand object is seen, by double reflection, in 

 the horizon mirror directly under the left- 

 hand object. With both objects in view they 

 are brought into coincidence with the tan- 

 gent screw. 



When the objects (signals) are definite 

 and readily visible, many observers use the 

 sextant without a telescope, especially if the 

 angle is changing rapidly or the sounding 

 vessel is unsteady. When the objects are 

 distant, indistinct, or indefinite, when the 

 angles are changing slowly, and particularly 

 where a small error in the angle will affect 

 the position considerably, a telescope should 

 always be used (see 5-40) . 



Little experience is required to measure 

 sextant angles between prominent objects 

 when the angle is changing slowly and the 

 observer's platform is steady. In hydro- 

 graphic surveying, the circumstances are 

 often reversed — angles change rapidly, ob- 

 jects are indistinct, and the survey vessel is 

 far from steady. Under such conditions, a 

 great deal of practice is required to enable 

 one to observe sextant angles quickly and 

 accurately. 



As a general rule, after taking an angle 

 and reading it, an observer should not move 

 the index arm until the position has been 

 plotted, so that he can verify the reading if 

 necessary. It is always good practice to 

 verify the reading of a sextant angle before 

 the arm is moved. 



The two angles of a three-point fix must 

 be measured simultaneously on signal, at the 

 word "mark." If one of the objects cannot 

 actually be seen or if they are not in coinci- 

 dence at the "mark," the angle must be re- 

 ported to the ofl[icer-in-charge as a "miss." 



3-14 Angles to faint objects. — When sig- 

 nals are faint, it is sometimes difficult to 

 reflect the right-hand object even though 



