Group of the Pleiades. 



277 



Table of the Tangent Correction. 



Dist. 



Corr. 



O 

 86 1 

 1242 

 1472 

 1647 

 1791 

 1914 

 2024 

 2123 

 2214 

 2297 



2375 

 2448 



2517 

 25S3 

 2644 

 2705 

 2762 

 2816 

 2869 

 2919 



11 

 — .OO 

 — .OI 

 — .02 



—•03 

 —.04 



— -°5 

 —.06 

 —.07 

 —.08 

 —.09 

 — .IO 

 — .11 

 — .12 



— •13 

 —.14 



— 15 

 —.16 



—.17 

 —.18 



—.19 



Dist. 



Corr. 



11 

 2919 

 2969 

 3016 

 3062 

 3107 

 3ISI 

 3*93 

 3233 

 3274 

 3313 

 3352 

 33^9 

 3426 



346i 

 3496 

 3531 

 3565 

 3598 

 3631 

 3663 

 3 6 94 



— 



11 



20 

 21 

 22 

 23 

 24 

 25 

 26 



27 



28 



29 

 30 

 31 

 32 



33 

 34 

 35 

 36 

 37 

 38 

 39 



The fifth column contains the position angle, exactly as given in 

 the observation books, where it is referred to the last image of the 

 central star as a zero point. The sixth column gives the final 

 corrected value of the position angle. This is obtained from the 

 previous column by applying first a correction of -f 270 , so that 

 the angles may be counted in the usual way from the north point 

 towards the point of greatest right ascension. The remaining cor- 

 rections are the zero correction (Section VI.), and the refraction 

 correction (Section III.). In addition to these, the Western im- 

 pressions need a further correction which is not necessary for the 

 Eastern impressions. When the plates were measured, the Eastern 

 impression of the central star was centred upon the position circle 

 of the micrometer. The impressions were then separately measured, 

 using both the longitudinal motion of the micrometer, and that at 

 right angles to it. It follows that the position angles for the 

 Western impressions have all been referred to the same zero as 

 those of the Eastern impressions. They must therefore all be cor- 

 rected by the amount that position angle 270 changes when the 

 point at which it is measured moves along the trail a distance equal 

 to the distance between the two impressions of the central star. In 



