DEPARTMENT OF MERIDIAN ASTROMETRY. 247 



The means of microscope readings have been computed and the 

 <;orrections for division error and error of runs applied. The corrected 

 zenith distances have been transcribed to the computation sheets, and 

 the corrections for inclination of the zenith distance wire, curvature, 

 ^nd the combined telescope and circle flexure have aheady been applied. 

 The inchnation has been determined by scattered observations ex- 

 tending over three and one-half years. The adopted value is 0*987 

 per 10^ of equatorial time, with a probable error of ±0''009. As few 

 bisections are ever made at a distance greater than 20^ from the central 

 wire, this is the most precise of the instrumental corrections. 



Computation of apparent place reductions is but little in arrears and 

 the computation of refractions is well under way. 



Further material has been gathered for the determination of the 

 causes underlying the changes in colhmation. This material lends 

 confirmation to the conclusion derived from an investigation by Mr. 

 Roy which indicates an effect similar to lost motion over a temperature 

 range of approximately 22° C. 



The critical examination of the San Luis microscope readings has 

 been completed, uncovering 922 errors of various kinds in the more than 

 350,000 readings. About a third of the errors were found in the com- 

 putation of means and the apphcation of division correction, but the 

 rest were errors of 5 or 10 seconds, and others of a miscellaneous nature. 

 The proportion of these errors which might otherwise have escaped 

 detection is sufficiently large to warrant a similar examination of the 

 microscope readings of the Albany observations. 



The completion of the examination of the San Luis circle readings 

 marked the forward movement in the reduction of the zenith distances. 

 A first approximation to a system for the reduction of the observations 

 has been made. With the Preliminary General Catalogue as a basis, 

 systematic corrections have been derived to reconcile it with the San 

 Luis results. Mr. Roy finds that not only do the three fundamental 

 observers differ from one another in the four positions of the instrument, 

 but that there are decided systematic differences in the results obtained 

 by each individual in the four positions of the instrument. The low 

 altitude of the pole prevented a thoroughly independent determination 

 of^refraction constants, but in order to allow to some extent for correc- 

 tions of this]|nature a tangent term was introduced. 



Table B. — Preliminary corrections to P. G. C. 



