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SCIENCE. 



[N. S. Vol. XII. No. 292. 



fors and at Arequipa ; and obsei-vations 

 cannot be simultaneous unless he can see 

 botli at once. "We see that the conditions 

 are extremely favorable. While Arequipa 

 is passing along in this region, near the sun- 

 set line, Pulkowa will be passing along in 

 this region, near the opposite horizon ; so 

 that we may combine observations made 

 at Arequipa early in the evening, near 

 the end of twilight, with those at Pulkowa 

 several hours later in local time, in the 

 latter part of the night, and they will be 

 nearly if not quite simultaneous. We can 

 thus obtain the maximum parallax, wMch 

 will be 48", with a little allowance formotion, 

 from the observations of those two stations. 

 Later in the season the Cape of Good Hope 

 will appear above the horizon, and then an- 

 other combination can be made, the circum- 

 stances will be less favorable. We might 

 have observations at Paris, Greenwich, Pots- 

 dam, etc. , but they will aiiord less favorable 

 means of comparison than Pulkowa and 

 Arequipa, which give the longest possible 

 base line. As to our own position, we can 

 observe in connection with Arequipa, but 

 not easily ; and in connection with the 

 Cape of Good Hope, but when Eros rises 

 with us it will set at the Cape of Good 

 Hope. On the whole we cannot too 

 strongly impress upon Professor Pickering 

 the desirability of getting at Arequipa the 

 best observations that can be made there, 

 for combination with all the other stations. 

 As to the method of making the observa- 

 tions, imdoubtedly we must depend upon 

 photography. There is no heliometer at 

 Arequipa, and at the Cape it is not certain 

 that it is available. It will not do to de- 

 pend upon the heliometer. We must de- 

 pend upon photography, and it is not easy 

 for one not an expert to make valuable sug- 

 gestions with regard to that. An additional 

 complication arises from the rapid motion 

 of the planet. It does not appear certain 

 that the planet can be photographed at all 



so as to be available for this purpose. It 

 must rest in one position long enough to 

 take a picture or there will be no trace of 

 it upon the plate. Professor Pickering says 

 that there will be no difficulty in getting a 

 trail ; but that will not answer the pur- 

 pose. In view of the fact that most of the 

 stars will be brighter than the asteroid, 

 photographs may be taken by focusing 

 upon the planet and taking a trail of the 

 brighter stars. But the difficulty remains of 

 getting the time from the trails. Another 

 difficulty is that we have no first-class pho- 

 tographic apparatus in the equatorial re- 

 gion, available for this purpose. I conclude 

 that the combination of Pulkowa and Hel- 

 singfors with Arequipa ought to be our 

 main reliance for obtaining the value of the 

 solar parallax. 



G. A. Peters. I should think that 

 owing to the fact that the planet is upon 

 the horizon during the observations for 

 parallax, the light would be much dimin- 

 ished, and that the photographic plan would 

 in that respect suffer a disadvantage in com- 

 parison with observations obtained visually. 

 Unless large telescopes are used, and the 

 atmosphere is extremely clear, there is dan- 

 ger that no observations can be obtained in 

 that way. 



Professor Newcomb. It is fortunate that 

 at the stations where the planet is near the 

 horizon the heavens are remarkably clear. 

 At Arequipa, Pulkowa and Helsingfors the 

 vapors near the horizon are but a minute 

 fraction of those in the clime of Washing- 

 ton. It is a curious meteorological fact 

 that here the vapor is generally so dense 

 that good seeing near the horizon is much 

 more rare than at the stations selected. 



C. C. Abbott. Would it not be well to 

 use isochromatic plates? 



Professor Newcomb. That is a detail 

 that we shall probably have to leave to 

 those who take the photographs. 



S. J. Brown. I have given this matter 



