448 KRETZ. 



Table VII. — Refraction Coefficients. 



Plate. 



M 



N, 



My 



N, 



I 



-1-0.000307 



— 0.000017 



-f- 0.00004 8 



-^0.000313 



II 



.000331 



.000022 



.000078 



.000319 



III 



.000287 



.000001 



.000003 



.000306 



IV 



.000339 



.000023 



.000091 



.000317 



V 



.000375 



.000021 



.000126 



.000327 



VI 



. 000424 



. 0000 I I 



.000168 



.000342 



VII 



.000362 



.000021 



.000119 . 



.000319 



VIII 



.000409 



.000013 



.000159 



.000334 



IX 



.000303 



.OCOOI7 



.000049 



.000308 



X 



.000320 



.000021 



.000072 



.000312 



XI 



.000347 



.000023 



.000100 



.000319 



XII 



.000384 



.000019 



.000134 



.000329 



XIII 



.000294 



.000015 



.000044 



.000302 



XIV 



.000312 



.000021 



.000068 



.000306 



Precession, Nutation, and Aberration. — None of these need 

 be taken into account. For as regards the first two, they, be- 

 ing due to motions of the earth, cannot a-ffect the configuration 

 of the stars, although they shift the axes of reference. The 

 absolute distances between the stars will, therefore, be unaffected 

 by these causes, but the differences in right ascension and dec- 

 lination will be changed. If, then, we compute the constants 

 by the method to be detailed later, /, e., by comparing certain 

 stars on the plate with their positions as obtained from meridian 

 observations reduced to some convenient epoch, then it is evi- 

 dent that the resulting right ascensions and declinations from 

 the plate will be referred to the same epoch, without the need 

 of applying any corrections for precession or nutation. For the 

 changes due to these causes consist partly in a motion of trans- 

 lation, and partly in a motion of rotation of the axes ; the former 

 will be entirely eliminated, while the latter will be included in 

 the orientation correction. 



Aberration may also be neglected. For Bessel ^ has shown 

 that it changes the position angles about a point equally, while 

 it affects all the distances, in whatever direction, by a constant 



1 Astronomische Unteisuchungen, Vol. I, p. 207 ; or Chauvenet, Astronomy, 

 Vol. II, p. 466. 



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