August 3, 1900.] 



SCIENCE. 



171 



THE ASTRONOMICAL AND ASTROPHYSICAL 

 SOCIEIY OF AMERICA. 

 II. 

 Photographs of Nebulae, and Star Clusters ob- 

 tained with the Crossley Reflector : By J. E. 

 Keeler. 



The photographs on glass were exhibited 

 by Professor Campbell, who called attention 

 to some of the difficulties which had to be 

 overcome in the use of the instrument and 

 to the important results established, that 

 nebulae as a rule are spiral in form and as 

 an exception irregular in form. 



Some New Spectroscopic Binaries : By W. W. 



Campbell. 



A list of half a dozen spectroscop'c bi- 

 nary stars, recently discovered by Messrs. 

 Campbell and Wright at the Lick Observa- 

 tory. (To be published in the Astrophysical 

 Journal.) 



A Simple Method of inserting the Comparison 

 Spectrum in Spectrographie Observations : 

 By W. H. Weight. Bead by Professor 

 Campbell. 



The length of the slit is limited by two 

 total reflection prisms mounted immediately 

 in front of the slit plate. The reflecting 

 edges of the two prisms are parallel to each 

 other and at right angles to the slit ; and 

 the distance between the reflecting edges is 

 the length of the slit. The artificial light, 

 from two sets of electrodes placed one near 

 each end of the full-length slit, enters the 

 slit through these prisms near their vertices. 

 The method possesses many advantages. 

 (To be published in the Astrophysical Jour- 

 nal.') 



Color Screens as applied to Achromatic Tele- 

 scopes : By George H. Peters. 

 Attention was called to the want of defi- 

 nition caused by the secondary spectrum in 

 large refractors. It was then shown how 

 this was removed at the TJ. S. Naval Ob- 

 servatory by adapting a cell filled with an 

 absorbing solution between the eye-piece and 



eye of the observer in the 26-inch telescope. 

 The thickness of the absorbing laj'-er is 

 about 6 mm., and the solutions used are 

 the following : 



1. Picric Acid and Chloride of Copper 

 dissolved either in water or alcohol, for 

 cutting out the red and blue light. 



2. Picric Acid dissolved in alcohol or 

 water. 



3. Bichromate of Potash in water. 



4. Chromate of Potash in water. 

 Solutions 2, 3 and 4 remove the blue and 



violet light. Alcohol is used in cold weather. 

 The absorption of the liquids is tested by a 

 direct vision spectroscope. 



Some of the benefits derived from the 

 color screen are : 



Improved definition on planets and satel- 

 lites and in certain classes of double stars. 



In observations of objects by day where 

 the blue light from the sky is removed. 



Professor See and Mr. Peters are jointly 

 credited with this device. (See Astronom- 

 ische Nachrichten, No. 3636. ) 



The Spectrographie Determination of the Mo- 

 tions of the Stars in the Line of Sight : By 

 W. W. Campbell. 



Complying with the Council's request for 

 a paper on this subject, the following points 

 were presented : 



The great interest taken in this problem 

 is attested by the fact that the largest re- 

 fracting telescopes in the United States, 

 France, England, Germany, Russia and 

 Africa are, or soon will be, largely devoted 

 to the determination of stellar velocities. 



It must not be assumed that, because a 

 36-inch objective receives nine times as 

 much light as a 12-inch, it will be able to 

 carry velocity determinations in that ratio 

 to the fainter stars. The larger telescope 

 is accompanied by stronger absorption of 

 the light, by larger stellar images (except 

 in perfect seeing), by greater difSculties in 

 guiding, etc. ; so that it is able to extend the 



