ASTRONOMY FOR 1889, 1890. 137 



bulletins which have a very important bearing upon astronomical pho- 

 toftraph}-, may be mentioned one by Dr. Bakhuyseu ou the measure- 

 ment of the plates by the method of rectangular coordinates, in which 

 he obtains star places compariug- favorably with those from meridian 

 observations. Dr. Vogel contributes one or two papers on the " res- 

 eaux " and the measurement of the plates, and Professor Kapteyn sug- 

 gests the expediency of taking the catalogue plates with three exposures 

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

 proper motions and parallaxes. Dr. Scheiner has an important paper 

 on the application of photography to the determination of stellar mag- 

 nitudes. 



In the fifth number of the Bulletin^ Professor Holden has two papers 

 on the photographic magnitudes of stars, and Mr. tSchaeberle one on 

 the same subject. There is also an abstract of Dr. Lindemann's photo- 

 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 i)hotographic plates. 



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

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

 bureaus will be established for measuring the negatives obtained at 

 observatories not provided with special apparatus for the purpose, and 

 photographic 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 i^hotographic record of solar phenomena, including solar 

 spectrum photography; a systematic description of the Imiar surface 

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

 satellites, of comets, meteors, and particularly of nebula?, clusters, and 

 of stellar spectra. 



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

 a bright background. Professor Holden cails attention to the fact that 

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

 of the objective; generally speaking it would be an advantage to dia- 

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

 ordinary observations during the day, a point of particular importance 

 in connection with meridian observations. 



Authoritative testimony as to the value of photography for obtaining 

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

 pared Dr. Gould's reductions of Rutherfurd's photograi)hs 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 j)roof that in i)hotography we have a means of in- 

 vestigation for micrometric work at least ou a par with any existing 



