ARTRONOMY FOR 1889, 1890. 137 



hiilu'tiiis wliicJi have a very imporhint bearing upon astronomicai [)lio- 

 toj^iiapliy, Miay l)e uientioned one by Dr. Baklinyseii on the measiue- 

 nieiit of the phites by tlie method of rectaugiihir coiirdiiiiites, in which 

 he obtains star places coniparing- favorably with those from meridian 

 ol)servations. Dr. Vo<;(4 contributes one or two papers on the " res- 

 eanx '' and the measnreaient of the plates, and Professor Kai)teyn sug- 

 j;'ests the expediency of taking- the catalogue ]>lates with three exposures 

 at intervals of six mouths, for the purpose of determining the stars' 

 I)roper motions and parallaxes. Dr. Scheiner has an important paper 

 on the application of photograi)liy to the determination of stellar mag- 

 nitudes. 



In the tifth number of the Bnlletin^Vrofessor Holden has two ])apers 

 on the photographic magnitudes of stars, and Mr. 8chael)erk^ one on 

 the same subject. There is also an abstract of Di'. Lindemauii's i)hoto- 

 metric determination of the star magnitudes of the Bonn Durchmus- 

 terung, and a paper by M. Trepied on the necessity of coming to some 

 understanding as to what is meant by stars of the 9th, 11th, and 14th 

 magnitudes on the photographic plates. 



The question of the reproduction of the plates and of the publication 

 of the map has been left open, but it is piobable that one or more 

 bureaus will be established for measuring the negatives obtained at 

 observatories not provided with special apparatus for the purpose, and 

 photogra[)hic copies of all plates will be preserved in selected places in 

 case of accident to the original negatives. 



A meeting of those interested in the various branches of astronomical 

 photography other than the chart was called by Messrs. Janssen and 

 Common in September, 1889. The chief matters for discussion being a 

 complete photographic record of solar phenomena, including solar 

 spectrum photography; a systematic description of the lunar surface 

 by photography on a large scale; photographs of pianets and their 

 satellites, of comets, meteors, and particularly of uebuhe, clusters, and 

 of stellar spectra. 



In discussing the theory of the photography of a star projected upon 

 a bright background, Professor Holden calls attention to the fact that 

 the most important factor is the ratio of the focal length to the aperture 

 of the objective; generally si)eaking it would be an advantage to dia- 

 phragm the objective during the day. This is also true with regard to 

 ordinary obser\ations during the day, a point of particular importance 

 in connection with meridian observations. 



Authoritative testimony as to the value of ])hotography for obtaining 

 accurate measures of star clusters is given by Dr. Elkin, who has com- 

 pared Dr. Gould's reductions of Rutherfurd's photographs of the Plei- 

 ades taken over 20 years ago, with the heliometer measures made at 

 Konigsberg and New Haven. The smallness of the probable error Dr. 

 Elkin regards as proof that in photography we have a means of in- 

 vestigation for raicrometric work at least on a par with any existing 



