228 CARNEGIE INSTITUTION OF WASHINGTON. 



and troublesome line investigated was X 5929.898, attributed by 

 Rowland to iron. Mr. Babcock has never been able to identify this 

 line in the spark, but has finally satisfied himself that it exists in 

 certain parts of the iron arc of the Pfund type, where it is seen most 

 easily very close to the negative pole, in coincidence with what appears 

 to be an oxide line. 



Horseshoe Vortices. 



Dr. Anderson has made some interesting investigations on horse- 

 shce vortices in liquids, in connection with the study of bipolar sun- 

 spots. A horseshoe vortex, developed in a tank of water by dipping 

 a semicircular plate of sheet metal into the liquid and quickly 

 removing it, travels in a straight line across the tank. On meeting 

 centrally a vertical plate of glass immersed in the liquid, with its plane 

 corresponding with the direction of motion of the vortex, the two 

 halves of the vortex, though divided by the plate, pursue their courses 

 without interruption. If one-half of the vortex be intercepted by a 

 second sheet of glass, standing at right angles with the first, and join- 

 ing it to the wall of the tank, the other half continues its motion to 

 the end of the glass sheet, where it suddenly develops an extension to 

 the surface of the liquid, thus returning to the horseshoe form. 



Professor Koch's Registering Micro-Photometer. 



The registering micro-photometer devised by Professor Koch and 

 described in "Annalen der Physik," 39, 705, 1912, was brought from 

 Munich to Pasadena early in August and mounted in the basement 

 of the office building for the purpose of determining its applicability 

 to the study of solar, stellar, and laboratory spectra. 



The spectrum photograph to be investigated is mounted on the 

 moving stage of a microscope, whose objective projects an enlarged 

 image of the spectrum upon a narrow slit placed in the upper focal 

 plane. The ocular of the microscope is replaced by a photo-electric 

 cell connected with a string electrometer. As the image moves across 

 the slit the variation in the intensity of the illumination of the photo- 

 electric cell causes corresponding fluctuations in the charge of the 

 electrometer and in the position of the string. An image of the 

 latter is projected upon a second sht placed immediately in front of 

 a moving photographic plate. The slit is perpendicular to the string 

 and to the direction of motion of the plate. The instrument thus 

 automatically registers upon the plate a curve showing the intensity 

 distribution of the lines of the spectrum examined. With Professor 

 Koch's instrument the scale of the curve may be either 7 or 46 times 

 that of the original spectrum. The practically instantaneous re- 

 sponse of the photo-electric cell and the high sensibility of the 

 electrometer make it possible to record with precision the variations 

 of density in the photograph. 



