156 RECORDS. 



SECTION OF ASTRONOMY, PHYSICS AND 



CHEMISTRY. 



January 5, 1903. 



Section met at 8:15 P. M., Professor Charles Lane Poor pre- 

 siding. 



The minutes of the last meeting of the Section were read 

 and approved. 



The following program was then offered : 



Harold Jacoby, Comparison of Astro-Photographic 

 Measures Made with the Reseau and without It. 



0. C. Trowbridge, Some Facts Regarding Persistent 



Meteor Trails — the Significance of Size, Color and 



Drift. 



Summary of Papers. 



Professor Jacoby's paper was as follows : 



The reseau method of measuring stellar photographs, as con- 

 sidered in the present paper, is similar to that in use in the 

 observatories participating in the photographic survey of the 

 heavens now in progress. The most important advantage of 

 this method of measurement is that it avoids almost altogether 

 the effects of possible 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 star-images ; in fact, in the case of close 

 clusters, it is well-nigh impossible on such plates to make sure 

 that the pairs of coordinates assigned to any star really belong 

 to the same object. All this possibility 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 disadvantages connected 

 with using the reseau. It is necessary, for instance, to make 

 certain assumptions, such as the following : 



I. That the division errors of the original reseau can be deter- 

 mined as accurately as those of a scale. 



