Sept. 1 3l 1883J 



NA TURE 



473 



detected from the observations of two or three evenings with an 

 indifferent instrument, if it relates to a single s>ar. 



The right ascensions determined at French observatories 

 exhibit systematic errors which follow nearly the same law as 

 those which characterise English observations. 



Distinctively German observati ns are nearly free from sys- 

 tematic errors. As far as they exist at all, their tendency is to 

 neutralise the errors inherent in distinctively English and French 

 observations. 



d. In the case of several catalogues residual errors of consider- 

 able magnitude remain after the systematic errors depending upon 

 the right ascensions have been allowed for. These errors are 

 found to be functions of the declination of the stars observed, 

 and without doubt have some connection with the form of the 

 pivots of the instrument with which the ob-ervations were made. 

 This statement holds true, especially with respect to the ob- 

 servations at Paris, Melbourne, and Brussels, between 1S5S and 

 1871 ; and to the Washington observations between 185S and 

 1861. 



e. The systematic errors which exist in observati ns previous 

 to 1S65 follow the same law and have nearly the same magni- 

 tude as the errors of the same class which are inherent in the 

 national ephemerides of the country in which they were made. 



The British Nautical Almanac and the Connaissance des Temps 

 are largely responsible for the perpetuation of this class of errors. 

 For a few years before and after 1S60 the ephemerides of the 

 Nautical Almanac were based upon the observations of Pond, 

 which contain large periodic errors. It is found that the errors 

 of this system have been transferred without sensible diminu- 

 tion to every catalogue in which the observations depend 

 upon Nautical Almanac clock stars. At English observatories it 

 has been the custom to correct the positions of the fundamental 

 stars by the observations of each successive year ; but this has 

 produced no sensible effect on the diminution of the periodic 

 errors, which belong to the fundamental system. The pen idic 

 errors of the American Ephemeris follow nearly the same law as 

 the errors of the Nautical Almanac, but their magnitude is some- 

 what reduced. The error of equinox is also less. 



Wolfer's Tab. Reg., upon which the Berlitur yahrbuch is 

 based, has no well defined systematic errors, and the correction 

 for equinox is nearly [he same in amount as in the American 

 Ephemeris, but with the opposite sign. The accidental errors 

 seem to be rather larger than in the system of the American 

 Ephemeris. 



f. A general estimate may be formed of the relative magni- 

 tudes of the errors of secondary catalogues by comparing the 

 average error for each star of the primary catalogue. The 

 numbers given below represent the average deviation for each 

 star, expressed in hundredths of seconds, after the various cata- 

 logues have been reduced to a common equinox : — 



Average error 

 fji each star. 



Argelander 1S30 ... l'l 



Pulkowa 1845 ... l'l 



Greenwich 1845 ... 20 



Greenwich 1S60 ... 20 



D'Agelet (liouldl 1783 ... 22 



Cape of Good Hope (Henderson)... 1833 ... 22 



Greenwich 1850 ... 22 



Greenwich 1 871 ... 22 



Pans 1867 ... 24 



Washington 1846-52 ... 25 



Struve 1830 ... 2-5 



Cape of Good Hope 1S56 ... 28 



Kadcliffe i860 ... 31 



Greenwich 1840 ... 31 



Bessel 1825 ... 32 



Pond 1830 ... 37 



Gillis 1840 ... 3-8 



Madras (Taylor) 1830 ... 3'9 



Cape of Good Hope (Fallow;) ... 1830 ... 39 



Radcliffe 1845 ... 45 



Armagh 1840 ... 5'o 



Piazzi 1800 ... 5*3 



Bessel's Bradley 1755 ... 79 



Lalande 1800 ... 132 



Lacaille 1750 ... 24*9 



It is obvious from these relations that previous to about 1S25 

 the magnitude of the accidental errors of observation, combined 

 with the errors of reduction, prevent any definite conclusions 



with respect to the periodic errors inherent in these early 

 observations. It is probable, also, that early observations of 

 stars of the eighth and ninth magnitudes are subject to a class of 

 errors peculiar to themselve-, the nature of which is now well 

 nigh impo-sible to determine. 



The system itic errors in declination which belong to the 

 various secondary catalogues named are even more marked than 

 those in right ascension. The experience of Pond in 1833 is the 

 experience ol every astronomer who has attempted to compare 

 observations of the same star made at different times, under 

 different circumstances, with different instruments, and by 

 different observers. He says : " With all these precautions, we 

 do not find by comparing the present observations with those of 

 Bradley made eighty years ago under the same roof, and com- 

 puted by the same table of refractions, that we can obtain by 

 interpolation any intermediate catalogue which shall agree with 

 the observations within the probable limits of error." 



We owe to the investigations of Auwers {Astron. Nachr., 

 Nos. 1 532- 1 536), the first definite system of declinations which 

 is measurably absolute in its character. Yet the deviations of 

 this system from that derived by the same author, but from 

 much additional data in Publication xiv. of the Gesellschaft, is no 

 less than i'2s. The present difference 1 utstanding between the 

 Pulkowa and the Greenwich systems at lo° south declination 

 is 1 '7s. 



Within the past five years the labours of Auwers, of Safford, 

 of Boss, and of Newcomb, have resulted in the establishment of 

 a mean system of declinations from which accidental errors may 

 be considered to be eliminated in the case of a large number of 

 stars ; but the different systen s still differ systematically inter se 

 by quantities which are considerably greater than the probable 

 error of any single position. 



When the discussion of the question of a uniform determina- 

 tion of all the stars in the northern heavens to the ninth magni- 

 tude was taken up by the Gesellschaft at its session in Leipzig in 

 1865, Argelander, who was then president of the Society, 

 appears to have been the only astronomer who had a clear 

 apprehension of the difficulties of the problem. He alone had 

 detee'ed the class of errors whose existence subsequent investiga- 

 tions have definitely established. He alone had found a well- 

 considered plan by which these errors might be eliminated, as 

 far as possible, from future observations. 



Argelander, however, always claimed for Bessel the fir t 

 definite proposal of the proposition under consideration (see 

 Astron. Nachr., i. 257). It was in pursuance of this plan that 

 the zones between - 15° and + 15° in declination were observed. 

 These zones were to form the groundwork of the Berlin charis ; 

 and Argelander, in the execution of the Bonner Durchmusterung, 

 simph carried out the second part of Bessel's recommendation. 



With the exception of the observations of Cooper at Markree 

 Observatory, and the charts of Chacornac, these two great works 

 — the second being a continuation of the first, under a better and 

 more feasible plan — are the only ones in existence which give us 

 any knowledge of the general structure of the stellar system. 



The observations of stars to the ninth magnitude, found in the 

 catalogues of Bessel, Lalande, and Piazzi, form the groundwork 

 of these charts. The coordinates in right ascension and declina- 

 tion of the stars found in these authorities were first reduced to 

 the epoch 1800 ; the resulting right ascension being given to 

 seconds of time, and the declination to tenths of minutes of arc. 

 With these places as points of reference, all other stars were 

 filled in, down to the ninth magnitude, by observations with 

 equatorial instruments. The work was divided into zones of one 

 hour each. Bremiker undertook five zones ; Argelander and 

 Schmidt, two ; Wolfers, three ; and Harding, two. The re- 

 maining zones were undertaken by different astronomers in 

 w idely separated localities. 



The work seems to have been performed with somewhat un- 

 equal thoroughness, some zones containing nearly all the stars to 

 the ninth magnitude, while ia others a large number of stars 

 having this limit in magnitude are wanting. 



The Durehmusterung, undertaken by Argelander at Bonn, 

 was a far more serious and well-considered undertaking. This 

 unequalled work consi-ts in the approximate determination 

 of the coordinates of 324,198 stars situated between - 2° and 

 + 90° declination. It includes stars to the 9*5 magnitude, the 

 coordinates being given to tenths of seconds of time, and the 

 declinations to tenths of minutts of arc. 



The first definite proposal of this work undertaken by the 

 Gesellscliajt, however, appears to have been made by Bruhns. In 



