MOUNT WILSON OBSERVATORY. 213 



Reference may be made to the following determinations of radial 

 velocity : 



Messier 3 (7 stars) — 190 km. 



N. G. C. 1700 +800± 



N. G. C. 3379 +850 



The two nebulae belong to the class of small objects which have for 

 a nucleus a condensation almost of stellar form. The traces of 

 outlying nebulosity are insufficient to give any definite information 

 concerning the spiral structure. 



A spectrogram of the Andromeda nebula was obtained by Mr. 

 Pease with an exposure time of 79 hours, the sHt being set along the 

 major axis. The multiple-comparison device was employed and com- 

 parison spectra were photographed at different distances along the 

 nebular lines. The measurement of the plate showed marked evidence 

 of rotation. At a distance of 150" from the nucleus, opposite points 

 in the nebula have radial velocities which differ by 170 km. For inter- 

 mediate points a linear relationship between velocity and distance is 

 indicated, the values being given by the formula 



y=-OASx-Sm 

 in which y is the radial velocity in kilometers and x the distance in 

 seconds of arc. A previous photograph taken with the slit set along 

 the minor axis showed no evidence w^hatsoever of rotation and thus 

 furnished valuable evidence for the accuracy of the results. The 

 radial velocity of the nucleus of the nebula derived from the two spec- 

 trograms is —306 Ion. 



PHYSICAL LABORATORY. 

 INSTRUMENTS. 



The chief addition to the laboratory fittings during the year has 

 been the installation of a very complete switchboard by means of which 

 excellent facilities of current-supply are obtained in all parts of the 

 laboratory. Thus alternating current at 52, 104, and 208 volts and 

 direct current from generators of 12 and 17 k. w., used either singly 

 or in series, are supplied to all the instrument pairs; 3-phase current 

 and special connections for the high-tension transformer and the large 

 electromagnet are also provided on the switchboard, the knife switches 

 of which have an interlocking system to prevent cross-connection of 

 different circuits. 



The repair of a sHght fire damage in the transformer room, appar- 

 ently resulting from an electrical storm, gave better fire protection 

 to the building and an improved arrangement of the high-voltage 

 connections. 



The large high-tension transformer has been entirely rebuilt. The 

 secondary voltage is now approximately 150,000, and tests showed 

 the insulation capable of standing an 8-inch spark in air. Four varia- 

 tions of the primary windings are available and all parts of the appa- 

 ratus are readily accessible for repairs. 



