Februaey 27, 1903.] 



SCIENCE. 



323 



G. H. Peters : ' The Photoheliograph of the 

 U. S. Naval Observatory; its Use and Defects in 

 Solar Photography.' 



Simon Newcomb: 'Statement of the Progress 

 made by the Watson Trustees in Computing 

 Tables of the Asteroids discovered by James C. 

 Watson.' 



A. S. Flint : ' Results of Meridian Observations 

 for Stellar Parallax made at the Washburn Ob- 

 servatory.' 



L. A. Bauer : 'Preliminary Summary of Mag- 

 netic Results obtained during the Recent Erup- 

 tion in Martinique.'' 



S. D. TowNLEY: 'The Light of the Stars.' 



ABSTRACTS OF PAPERS. 



Comparison of Astronomical Photographic 



Measures Made with the Beseau and 



without it: Harold Jacobt. 



The reseau method of measuring stellar 

 photographs, as considered in the present 

 note, is similar to that in use by the ob- 

 servatories participating in the photo- 

 graphic survey of the heavens now in prog- 

 ress. The most important advantage of 

 this method of measurement is that it 

 avoids almost altogether the effects of pos- 

 sible contractions or expansions of the sen- 

 sitive film during development; and to this 

 advantage has been joined another of a 

 practical character which was perhaps not 

 foreseen by the originators of the reseau 

 method. It is found most confusing to 

 measure plates having nothing on their 

 surfaces but stars-images; in fact, in the 

 case of close clusters, it is well-nigh impos- 

 sible on such plates to make sure that the 

 two coordinates assigned to any star really 

 belong to the same object. All this pos- 

 sibility of confusion disappears, however, 

 with reseau plates, as it is easy to keep all 

 measures in order by considering each little 

 square by itself. 



As usual, there are compensating disad- 

 vantages connected with the reseau. It is 

 necessary, for instance, to make certain as- 

 sumptions, such as the following: 



1. That the division errors of the orig- 

 inal reseau can be determined as accurately 

 as those of a scale. 



2. That the photographic copy of the 

 reseau, as it appears on the star-plate, 

 really reproduces exactly the division 

 errors of the original. 



3. That the bisection of the photo- 

 graphed reseau lines on the star-plate can 

 be made with a microscope as accurately 

 as the lines of a scale can be bisected. 



It is of course possible to discuss each of 

 these assumptions separately ; but in the 

 present note I shall consider one simple 

 experiment only. This consisted in meas- 

 uring a couple of Pleiades photographs 

 twice, once by the reseau method, and once 

 with a metallic scale. A simple compari- 

 son ought then to show how far the two 

 methods of measurement differ in their re- 

 sults. Seventy-five stars were observed in 

 each case, and the same stars were used. 

 The first plate was made at Paris, January 

 14, 1901, and the 'probable discordance' 

 between the two methods of measurement 

 was ± 0".ll. No corrections were applied 

 for possible division errors of the Paris 

 reseau, as none have been published, 

 though the MM. Henry have satisfied 

 themselves that the Paris reseau errors are 

 inappreciable. The second plate was 

 made at Helsingfors, December 12, 1900, 

 and gave a probable discordance of 

 ± 0".22. In this case, the measures were 

 corrected with Donner's division errors, 

 but these are not large enough to affect the 

 result appreciably. In both cases, meas- 

 ures made with the metallic scale were 

 corrected for the division errors deter- 

 mined at Columbia University. The larger 

 discordance in the case of the Helsingfors 

 plate is probably due to the less well de- 

 fined character of the photogi-aphed reseau 

 lines. In many cases it is impossible to 

 bisect these lines under the microscope any- 



