CHAP. XIII.] OBSERVATIONS FOR ERROR 303 



shades on the sextant. Moreover, the suns having been once 

 equahzed as to briUiancy with one eye-piece, by moving the 

 up-and-down piece screw they will remain equal, no matter 

 what shade of eye-piece we use ; but with the hinged shades, 

 the position of up-and-down piece, which equalizes the suns, 

 as seen tlirough one set of them, will be different to that 

 required for others, besides the possibility of error thus intro- 

 duced. 



The suns should be as dark as possible. If too light shades Suns 



1 . -Ill r XI 1- 1- should not 



are used, the irradiation spoils the sharpness of the limb. be too 



Use the eye-piece with the greatest magnifying power, as Bright, 

 it much facihtates correct contacts. f^iSgeas 



Great care must be taken in setting the vernier, and we possible, 

 must see that the tangent screw at the commencement of each Setting 

 set is run back to its full extent, so as to avoid risk of being vernier. 

 " two blocks " in the middle of the set, and so probably lose 

 an observation. 



After bringing the zero of the vernier into what we believe 

 to be coincidence with the minute of arc required, glance 

 right and left to see that the marks on vernier and arc are 

 displaced in a symmetrical manner on either side. The eye 

 will easier catch any inaccuracy in the setting by this 

 means. 



In setting, turn the tangent screw for the final adjustment 

 the same way both forenoon and afternoon. Thus, if with 

 altitudes increasing the tangent screw is turned to the right; 

 to attain coincidence, with altitudes diminishing the vernier 

 must be set back below the required altitude, so as again to 

 turn the screw to the right for final adjustment. This tends 

 to eliminate error from slackness of screw. 



Some observers, after giving a preliminary "Ready" at Warning 

 the commencement of each set, give no warning after, and 

 simply " Stop " at each observation. With very careful 

 time-takers this is sufficient, but experience of human nature 

 leads us to say that it is better to call " Ready " about three 

 seconds or so before each " Stop," and thereby avoid all 

 chance of the time-taker having his eye and ear off the 

 watch. 



If a chornometer fitted for use w ith a chronograph is avail- Use of the 

 able, the accuracy of the observations will be much improved, graph. 



