ASTRONOMY. 115 



ASTRONOMICAL PHOTOGRAPHY. 



The improvemeuts iu astronomical photography during the past two 

 years, following the introdaction of the modern dry plates, have at- 

 tracted wide-spread attention, and the great merits of the new method 

 scarcely call for any exaggeration iu order to establish photography 

 permanently as a means for astronomical research. We find Greenwich, 

 Harvard, Paris, Cape of Good Hope, and Lick taking steps to make 

 stellar photography a part of their routine work, and arrangements have 

 been made by Admiral Mouchez for holding an international conference 

 at Paris in April, 18S7, for the purpose of elaborating a plan of co-op- 

 eration in i^hotographing the whole sky. It is hoped that ten or twelve 

 observatories wiH be ready to co-operate and that all will be supplied 

 with instruments of the same power, so that the work will form a homo- 

 geneous whole. It will require 11,000 plates of 4° each to cover the sky, 

 and ten years will probably be necessary for the completion of the un- 

 dertaking. 



Stellar i)kotograpliy at the Paris Observatory. — An article in Nature 

 (jMay 13, 188G), which gives a wood-cut of the apparatus used by the 

 Messrs. Henry, gives also the following table of the time of exposure re- 

 quired (with the Mouckhoven gelatino-bromide plates) to obtain stars 

 of different degrees of brightness : 



Magnitude. Time of exposure. 



1 0^ 005 



2 . 013 



3 0.03 



4 . 08 



5 0.2 



6 The limit of magnitude visible to naked eye 0.5 



7 1.3 



8 3 



9 8 



10 ) £ 20 



11 > Mean magnitude of the asteroids < 50 



12 ) ^ 2"^ 



13 5 



14 i:] 



15 ? 



, ,. > The smallest stars visible iu large telescopes I'' 23 



These figures represent a minimum. To secure good reproductions 

 on paper the time of exposure would have to be increased threefold. A 

 two hours' exposure gives stars much fainter than Herschel's debilissima. 



The Henrys have successfully,;; photographed the clusters in Hercules, 

 Sobieski, Ophiuchus, and Perseus, and the major planets. They have 

 obtained the trail of an eleventh- magnitude asteroid — a fine line among 

 the stellar points. The new method seems well adapted, also, to the 

 search for a trans-Neptunian jilauet. 



The observatories at Algiers and Rio Janeiro are to be supplied with 

 instruments similar to those at Paris, and an equatorial coude of 0.6 

 meter (24 inches) apertue provided with a photographic objective is 

 to be constructed for the Paris Observatory, to test the adaptability of 

 this form of instrument for photographic work. 



