250 SCIENCE PROGRESS 



investigation, and in the other only one image. The positions 

 are called the " Single " and " Double Order " positions re- 

 spectively, and the intensities of the images in the two positions 

 differ in a known ratio, which is exactly what is required for 

 the half- width measurements. 



The experimental procedure is simply to take two photo- 

 graphs of a line on the same plate, with equal exposures, but 

 with the echelon set in the two different positions. Six micro- 

 meter settings are then made on the edges of the images, and the 

 half- width is calculated. The table shows results obtained by 

 this method. 



Calculated half -width. 

 Line. Observed half-width. For H atom. For Hg molecule. 



X 6018 0'034A. 0'046A. o-o32A. 



X 6028 o*o33A. 0-046A. o-032A. 



X 6225 0-035 A. o*048A. 0-034A. 



The lines measured were chosen from the more important of 

 the physical groups distinguished above, and the method was 

 tested by applying it to a helium line, where the mass of the 

 radiating particle is known. It is fairly safe, therefore, to 

 assume a molecular origin for the secondary spectrum, in 

 attempting to treat it theoretically. 



In conclusion, reference must be made to some very curious 

 observations on the distribution of the hydrogen spectra in 

 vacuum tubes. Prof. Wood, in the attempt to extend the 

 Balmer series beyond its normal laboratory limits, prepared 

 a vacuum tube with a very long capillary. On filling the tube 

 with hydrogen, the discharge was at first bright red throughout 

 the capillary, and the predominant feature everywhere was the 

 Balmer series. After running the tube with a powerful current 

 for some time, its appearance changed. The discharge became 

 white at the ends of the capillary, and, in the spectroscope 

 showed mainly the secondary spectrum. The centre of the 

 tube was still ruddy owing to the intensity of Ha, but the white 

 discharge gradually encroached upon it, and finally filled the 

 whole tube. 



Prof. Merton at about the same time observed a peculiar 

 phenomenon in helium tubes containing — as they usually do — 

 a trace of hydrogen. With a normal uncondensed discharge, 

 the Balmer lines and the secondary spectrum, if it is visible, are 

 equally intense all along the capillary. If a condenser is intro- 

 duced momentarily into the circuit, on removing it again the 

 distribution of the hydrogen lines is found to be disturbed. They 

 show only at the ends of the capillary, but in a few seconds they 

 work their way back to the centre. Further investigation 

 revealed that the spectra of elements other than hydrogen. 



