﻿544 Prof. R. W. Wood on Atomic Hydrogen 



Pyrex glass has been found better than soft glass, but 

 quartz appears to be no better than pyrex. 



The fact that we have only atomic hydrogen in the central 

 part of a long spectrum tube, even during the brief moments 

 when no current is passing (between the half -cycles), makes 

 it appear probable that the refractive index of the gas in the 

 atomic condition can be determined by introducing the tube 

 into one path of an interferometer, and illuminating the 

 instrument with flashes of light, during the moments when 

 the current is not passing, by means of a disk perforated 

 with two slots rotated by a synchronous motor. This 

 experiment will be tried in the autumn. 



Part II. The B aimer Series. 



In continuing the work on the Balmer spectrum of 

 hydrogen, tubes of pyrex glass have been used exclusively. 

 The aluminium-foil electrodes were attached to tungsten 

 wires, which fuse easily into pyrex. As there is apt to be a 

 slow capillary leak along the tungsten wire, a drop of sealing- 

 wax was always melted around the wire on the outside of the 

 bulb. These tubes, if made of carefully cleaned glass, will 

 come into the " black stage " (showing the Balmer lines on 

 a black background when viewed through a direct-vision 

 prism) after ten or fifteen minutes' operation. 



The lines of the Balmer series were photographed in the 

 2nd- and 3rd-order spectrum of a very perfect 7-inch plane 

 grating (temperature controlled to o, l by a thermostat) with 

 an objective of 20-foot focus, H a , H^, and H y all showing us 

 clearly separated doublets. The series was recorded with 

 this apparatus as far down as the 18th line with an exposure 

 of only 12 hours. Only a few minutes were required for the 

 plates showing H v -H e . 



Curtis, using a concave grating of rather short focus, with 

 an exposure of 5 hours obtained only the first six lines of 

 the series. When we consider that the sixth line has an 

 intensity about 4000 times as great as that of the 18th line, 

 the enormous intensity and efficiency of the long end-on tube 

 is at once apparent. The tube was excited by a large 6000- 

 volt transformer. 



Photographs were also made in the 5th-order spectrum of 

 a plane grating, in combination with a collimator and Cooke 

 portrait lens of 1-metre focus. A screen of glass coloured 

 by nickel oxide was used to cut out the overlapping green of 

 the 3rd order and violet of 5th order, which covered the 

 region of the last lines of the Balmer spectrum* This glass 

 is opaque to all visible light except the extreme red, and is 



