August 3, 1900.] 



SCIENCE. 



187 



some attention. It seems to me that mi- 

 crometric observations could be carried on 

 simultaneously, especially in the United 

 States. We have Pulkowa, Strassburg, 

 Charlottesville, Evanston, Madison, Prince- 

 ton, Washington, and the Lick Observa- 

 tory, and the Yerkes, where work can be 

 advantageously done with large telescopes. 

 While the factor of the parallax is very 

 much smaller in the case of simultaneous 

 micrometric observations, the main thing 

 seems to be to secure the necessary co-opera- 

 tion, and to prepare a careful plan before- 

 hand. The motion of Eros is so large that 

 the observations will have to be very nearly 

 simultaneous. The difference in the factor 

 for parallax between Pulkowa and Wash- 

 ington is about 1.1. It is not much greater 

 between Pulkowa and the Lick Observa- 

 tory. It is 1.92 at Charlottesville; 1.26 

 for the Lick Observatory, 1.13 for Wash- 

 ington, and about the same for Princeton ; 

 so that by simultaneous observations be- 

 tween Pulkowa, Strassburg, Greenwich, and 

 the observatories in the United States, we 

 may obtain a parallax of about 1-5" between 

 the Ist of October, and January 16th. My 

 impression is that stars should be selected 

 not more than 60" from the planet, and that 

 they should be compared at nearly the same 

 Greenwich mean time at all the stations. 

 In this way observations may be obtained 

 extremely valuable for determining the 

 parallax. I am not certain but that the 

 planet can be photographed to advantage : 

 but it would require a long time, and the 

 motion of the planet would be uncertain ; 

 so that the advantage I think would be out- 

 weighed by the accuracy with which micro- 

 metric observations could be made ; and if 

 referred to the same stars, eliminating the 

 motion of the planet, we might expect pre- 

 cision. The position of the planet can be 

 ascertained closely enough, and its parallax 

 known so nearly that we can measure it to ad- 

 vantage in telescopes varying from 16 to 40 



inches in aperture in the United States. I 

 had started to draw up a letter to be sent to 

 the directors of the observatories of Pulkowa, 

 Strassburg, and Greenwich in Europe, and 

 in this country at Charlottesville, Prince- 

 ton, Madison, the Yerkes and the Lick, 

 asking that co-operative work might be car- 

 ried on, and asking for a selection of the 

 stars. If we assume 60" for the greatest 

 distance from the planet, and a parallax 

 varying from 15" to 30", this will give a dis- 

 tance from 60" to 90" ; and you can go down 

 to the 10th magnitude, the magnitude of the 

 planet, and find in the field a number of 

 suitable stars, which should be symmetric- 

 ally situated in rectangular co-ordinates. 

 In this way it seems to me that we can ob- 

 tain from these large telescopes observations 

 of as great accuracy as those obtained by 

 the photograph, although the distance may 

 be twice as gi-eat and the factor twice as 

 great. I do not see that anything practical 

 can be done by this association. It must 

 remain for some one to take the initiative ; 

 and there ought to be no difficulty in get- 

 ting suggestions to perfect such a simple 

 plan, such as may result in saving the day 

 for the observations of Eros. Photographic 

 observations would have to be confined 

 largely to Pulkowa, Berlin, or Potsdam, 

 and Arequipa, the Cape of Good Hope will 

 not come in until along towards the end of 

 the opposition. It seems to me therefore 

 that micrometric observation, under these 

 circumstances will be a matter of consider- 

 able importance. 



Professor Newcomb. We want the stars as 

 nearly of the same magnitude as possible. In 

 a space 60' square, the mean distance of the 

 stars would be about 6'. The planet moves 

 about 2" of arc iu a minute. An observer 

 making a careful micrometric measure must 

 take at least ten or twenty seconds to make 

 up his mind about the bisection. During 

 10 seconds the planet will have moved over 

 a space of y of arc, a quantity not appre- 



