DEPARTMENT OF MERIDIAN ASTROMETRY. 177 



Catalogue =-^0".87 gives a negative correction to Preliminary General 

 Catalogue. Only a few stars are common to Cohn and Preliminary General 

 Catalogue, however; so this result has trifling weight. Such as it is, it applies 

 to declination +39°, the latitude of the international stations, but, as will 

 appear, the correction there is at least as great as at the equator. Newcomb 

 found +0''42 from the declinations of the planets as the correction to the 

 Northern Boundary Catalogue, which is corrected +0''25 by Preliminary 

 General Catalogue in the belt from +25° to —24°. This leaves a correction 

 of +0''17 to Preliminary General Catalogue. 



The solution of the question rests, in the last analysis, upon meridian 

 observations. It seems natural to suppose that at an observatory where 

 declinations are measured on the same plan, with the same instrument, and 

 reduced in the same manner over a period of many years, the systematic 

 errors would remain sensibly constant, and therefore the motions would not 

 be affected by them. The results do not substantiate this reasonable expecta- 

 tion. In the first place, at no station has there been any such consistency 

 for a long enough time. The nearest approximations are found at Greenwich, 

 Pulkova, and the Cape. 



An examination of the observations with reference to this problem can 

 conveniently be made through the systematic corrections to declination 

 dependent on declination as given in the Preliminary General Catalogue, 

 supplemented by Mr. Roy's derivation of the systematic corrections to the 

 more recent catalogues. In addition, corrections for Cape 1914 are to be 

 found in the introduction, and Mr. Raymond has determined those of the 

 Pulkova 1915 catalogue of declinations. According to these, Greenwich 

 in the Pond period gave corrections to the Preliminary General Catalogue 

 of about a second at the equator. In the intermediate periods this fell to 

 a small and sometimes negative value, and in 1910 changed back to a value 

 of about +0''5. These observations do not, therefore, give a straight line 

 when plotted with the time as an argument. The other series show similar 

 peculiarities, only less striking. These variations seem to be due largely to 

 changes of methods, particularly in the use of the refraction with which 

 the observations are reduced. Indeed, the major anomalies can be confi- 

 dently ascribed to refraction. Pond used Bradley's refraction constant. Airy 

 changed to Bessel's second value, that of the Tabulce Regiomontance, which 

 is much larger, and this was used, except for 1872, until recently, when the 

 1910 catalogue was reduced with the Pulkova refractions. 



Correcting for these changes, the agreement is greatly improved. The 

 mean of the three series indicates corrections to centennial proper-motions 

 in declination of the order of 0''4 near the equator. Pulkova gives +0''6 

 or 0T7, Greenwich about half as much, and Cape less than +0''l. The cor- 

 rection does not follow the cosine of the declination. Its maxima are about 

 +0T6 at or near declination +30° and —30°, with smaller values between 

 and tapering rapidly to the polar regions, where it is small. 



Mr. Raymond has also begun to treat some more recent catalogues in 

 pairs according to the method outlined by L. Boss in Astronomical Journal, 

 No. 23, p. 119. This work has gone far enough to indicate that the large 

 positive corrections to the declinations of the Preliminary General Catalogue, 

 given by some of the recent catalogues, are due in a measure to refraction. 

 The correction to the centennial proper-motion in declination will probably 



