March 21, 1901] 



NA TURE 



503 



can only hope that the favourable presage will be realised. The 

 probable limit of error is based upon a preliminary inquiry 

 due to M. Hermann Struve, of Konigsberg, who has found 

 that the probable error of a single complete micro- 

 metrical measurement is ±o"'077, and such an error would 

 introduce no greater uncertainty into the parallax than o"'03, a 

 most satisfactory result for one night's determination. Such a 

 favourable result, however, implies (l) that we are in possession 

 of the accurate diurnal motion of the planet ; (2) that no error 

 exists in the relative position of the stars of comparison, and 

 (3) that every source of systematic error has been eliminated. 

 It is not unimportant to observe in this connection that the 

 motion of the planet itself in one second of time can amount to, 

 and even exceed, o"'03 in the arc of a great circle, no incon- 

 siderable fraction of the total error found by M. Struve. The 

 actual epoch of exposure, with a rapidly moving shutter, would 

 probably be known to much less than a second of time, but the 

 proper moment to assign to the formation of the image seems 

 to be open to more doubt. A question of very similar import 

 has been discussed at Paris by M. Henry, and has been reported 

 upon. This has reference to the formation of the trace of 

 the planet on the sensitised film, when the equatorial is 

 driven to sidereal time by means of a star. M. Henry photo- 

 graphed a region of the sky with an exposure of three minutes, 

 in which an acceleration and retardation of three seconds was 

 alternately given to the driving clock. Two other exposures 

 were made on the same plate in the reversed order, and the 

 differences of right ascension of the centres of the traces were 

 measured. The mean of the differences of the measured 

 distance with clock accelerating was compared with the mean of 

 the distances clock retarding, in groups according to magnitude, 

 with the following result : — 



Number of Mean mag- Mean diff. 

 stars. nitude. of distance. 



Prob. error of Prob. error of 

 the mean. a distance. 



The large probable error in the third group is quite suffi- 

 ciently explained by the faintness of the stars and the shortness 

 of the exposure, but we seem to be in presence of errors of 

 quite the same order of magnitude as those found by M. Struve. 

 Certainly one of the most interesting of the results that will 

 proceed from this elaborate programme will be the relative cer- 

 tainty and freedom from systematic errors of the various methods 

 of observing. 



This memoir or report also contains, besides an ephemeris 

 of the planet supplied by M. Millosevich and a table of star 

 constants applicable to the stars used in the discussion of the 

 photographic plates, a memoir by Mr. Comstock on the com- 

 putation of refraction in the direction of the diurnal motion of 

 the planet. Other points which have been discussed are crowded 

 out of the present number, but enough is given to assure us that 

 the International Committee forms a centre of activity calcu- 

 lated to attract the energy and the enterprise of all the co- 

 operating astronomers. But it is impossible to anticipate, as 

 the result of so much labour, anything more than an academic 

 interest. Based upon the constants employed in the reduction 

 and an assumed figure of the earth, the resulting parallax will 

 represent the best value procurable from an isolated inquiry ; 

 but in view of the solemn acceptance of the value 8" "So for the 

 solar parallax at the Paris Congress in 1896, it seems extremely 

 improbable that the various national ephemerides will make any 

 alteration in a value which has been so recently introduced. 



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NO. 1638, VOL. 63] 



