January 3, 1913] 



SCIENCE 



31 



two, in that all elements used were determined by 

 the same person and the same instrument. 



The Attraction of Sun-spots for Prominences: 



Frederick Slocum. 



In the autumn of 1910 a large group of sun- 

 spots passed several times across the face of the 

 sun. On its first passage, from August 2 to 

 August 15, it was given the Greenwich number 

 6,874. 



Active prominences were observed in the imme- 

 diate vicinity of the spot group from August 2 

 to November 5, but the best displays occurred on 

 the west limb on October 8, extending from 

 Lat. -j- 6° to — 37°, and on the east limb on 

 October 22, extending from Lat. + 12° to — 36°. 

 Slides of calcium spectroheliograms of the spot, 

 the surrounding flocculi and the attendant promi- 

 nences were shown. 



The prominences were pouring from both sides 

 apparently right down into the spot. Points that 

 can be identified on two or more plates give veloci- 

 ties along the apparent trajectory up to 110 km. 

 per second, and show a marked acceleration to- 

 wards the spot. The conclusion is that some sun- 

 spots exert an attraction for the material of which 

 prominences are composed. 



Stellar Parallaxes from Plates made with the 

 Forty-inch Refractor of the Terkes Observa- 

 tory: F. Slocum and S. A. Mitchell. 

 The present parallax program contains about 

 150 fields, including (a) stars with large proper 

 motion, (6) stars with large radial velocity, (c) 



binaries, visual and spectroscopic, (d) selected 

 stars of the different spectral types, (e) novae, 

 and stars in any way peculiar, (/) the Kapteyn 

 zone + 45°. The following parallaxes have re- 

 cently been determined: 



The average number of plates used for each of 

 the above determinations is eleven, the average 

 number of comparison stars, five, and the average 

 magnitude of the comparison stars, 9.6. Numbers 

 3, 4, 5, 9 and 10 were measured and reduced by 

 Professor S. A. Mitchell, of Columbia University, 

 and numbers 1, 2, 6, 8, 11 by Professor Slocum, 

 and number 7 by Miss M. M. Hopkins, of Smith 

 College. 



The Spectrum of Nova Geminorum No. 2 on 



March 13, 1912: E. H. Cuetiss. 



The Detroit Observatory slit spectrograms of 

 Nova Geminorum II. secured on March 13, 1912, 

 show characteristics which have important bearing 

 on the nature of the spectral changes of a nova 

 as it approaches its maximum radiance. The 

 nova's spectrum on this date strongly resembles 

 that of March 15, when the nova maximum type 

 was generally recognized, the peculiarities of the 

 light on the earlier date being strongly accentu- 

 ated two days later. The conclusion is suggested 

 that the peculiarities of the nova type appeared 

 in the spectrum of this star earlier than has been 

 supposed. 



The Plane Grating Spectrograph for Stellar 



Work: J. S. Plaskett. 



This paper gives a short account of some pre- 

 liminary tests on the use of a plane grating as the 

 dispersion piece of a stellar spectrograph. The 

 grating with ruled surface 2J by 35 inches, 15,000 

 lines to the inch, was specially ruled to give great 

 concentration and it is estimated that 50 per cent, 

 of the incident light is diffracted into the first 

 order on one side. The spectrograph has been 

 tested only in the Littrow form, with a 2i-inch 

 Brashear Triplet of 37J-inch focus, giving a linear 

 dispersion of about 17.5 A. per mm. at Hy. It 

 gives beautiful definition and a field very nearly 

 flat over the range used from X4800 to X3500. 

 The relative intensity of star spectra obtained 

 with this instrument, and the Ottawa three-prism 

 spectrograph with a camera objective giving the 

 same dispersion at H is as follows: Region 

 X4700-X4300, prismatic about twice as intense 

 as diffraction spectrum. Region X4300-X4209, 

 spectra of nearly equal intensity. Region X4150- 

 X4100, diffraction two to three times as intense 



