60 Rutherfurd Photographic Measures of 



East and West zero-corrections computed for each by the 



formula * 



v — ^kz tan 6 — y -\- x 



in which v is the zero-correction to be added to all observed posi- 

 tion angles of each plate. 



In the nest column are the special corrections f required by the 

 position-angles of the Western impressions in consequence of 

 using the same zero-point in measuring both Eastern and West- 

 ern impressions. | The sum of these two columns is then given in 

 column six, which, therefore, contains the final correction as 

 actually applied in the reductions. 



8. In Table IV. is given the tangent correction. This is always 

 negative and its unit is .oooi divisions of the micrometer. It 

 has been computed by the formula : 



Correction = — Js 3 d 2 sin 2 1" = [1.7887,, ] s 3 



where s denotes the distance in divisions of the glass scale and d 

 is the value of one division of the scale in seconds of arc. 



Table v.— Measures of Distance. 



9. The first column contains the numbers of the stars in order 

 of right ascension and also in parentheses, for convenience of ref- 

 erence to the original measures and plates, are the numbers as as- 

 signed by Rutherfurd. The number of the plate is given in col- 

 umn two, after which follows the observed distances for the Eastern 

 and Western impressions. The numbers set down are the frac- 

 tional part of the measured distance expressed in divisions of the 

 glass scale, the whole number of divisions being ordinarily the 

 same as that given in the final corrected distance. Where there is 

 a change of .8 or .9 in the observed distance, it is an indication of 

 a change of a unit in the whole number of divisions in passing 

 from the observed distance to the corrected mean. In columns 

 five, six and seven are placed the corrections as applied for refrac- 

 tion, § aberration || and scaled respectively; these, with addition of 



* Annals N. Y. Acad, of Sci., Vol. VI., p. 272. 

 t Ibid., p. 278. 

 rflbid., p . 240. 

 \ Table II and Paragraph 2. 

 || Table III and Paragraph 4. 

 If Pleiades, pp. 242-251. 



