250 



CARNEGIE INSTITUTION OF WASHINGTON. 



Although the negative is somewhat underexposed, it yields two interest- 

 ing results : 



(1) The nebula has a large negative velocity, of the order of —600 

 km. on the west side and —1,000 km. on the east side, at distances in 

 each case of about 60'' from the nucleus. 



(2) The doubhng of the nebular lines first observed by SHpher seems 

 to be fully confirmed. The separations for the lines X3728 and X5007 

 are 29 and 41 a, respectively, values which indicate a direct propor- 

 tionality between separation and wave-length. 



Determinations of radial velocity have been made for the following 

 star-clusters : 



PHYSICAL LABORATORY. 

 INSTRUMENTS. 



The chief additions to the equipment of the Physical Laboratory 

 have been a short-focus plane-grating spectrograph, designed by Mr. 

 Anderson; a small electric furnace, designed by Mr. King for use 

 between the poles of the large Weiss magnet ; a Langmuir high-vacuum 

 pump with auxiliary pump; and speculum- metal mirrors and other 

 accessories for the interferometer spectrograph. 



The short-focus plane-grating spectrograph, designed especially 

 for the study of faint light-sources, has for collimating lens a Brashear 

 doublet of 5 inches aperture and 60 inches focal length, and for 

 camera lens a Bausch and Lomb Tessar Ic of 3.5 inches aperture 

 and 15.75 inches focal length. The 4-inch Rowland plane grating is 

 used at a constant angle with the axis of the camera lens. When this 

 angle, as is usually the case, is about 70°, the dispersion is nearly three 

 times as great as it would be with a nearly normal spectrum. The 

 camera, with the grating, is capable of rotation about a vertical axis 

 through the grating, the range being about 30°. The collimator can 

 also be rotated in the same way through an arc of 00°, which makes 

 it possible to use any order of spectrum up to the fourth. The plate- 

 holder takes plates 2.5 by 4 inches and is so mounted that, in addition 

 to the focusing adjustment, it can be tilted, and also given a vertical 

 motion, in order to place several exposures on the same plate. In the 



