470 Prof. R. W. Wood on the 



of the light emitted by the vapour when stimulated by 

 monochromatic radiations and white light. 



A number of improvements have been made in the 

 arrangement of the apparatus, and it is now possible to show 

 the resonance spectrum to a class in optics by means of a 

 commercial Cooper-Hewitt lamp (in glass) and a small 

 pocket spectroscope. A glass tube 2 or 3 cm. in diameter 

 and 30 cm. long is used. One end is blown out into a 

 small bulb of a diameter only slightly larger than that of 

 the tube, care being taken to get the glass as free from striae 

 as possible. It is especially important to get rid of the small 

 drop of glass which usually forms at the point where the 

 tube is drawn down. The tube is now carefully dried, and 

 after the introduction of a few small crystals of iodine, 

 drawn down .to a thick-walled capillary at the other end, 

 highly exhausted with a mercury pump, and sealed. It is 

 then mounted alongside of. and almost in contact with, the 

 Cooper-Hewitt lamp, the direct radiation unconcentrated by 

 lenses being used. On looking down the axis of the tube 

 through the bulb the yellowish fluorescent light is seen, and 

 the spectroscope exhibits the discontinuous resonance spectrum 

 composed of isolated bright lines. If the light from the 

 carbon arc, or still better sunlight, is concentrated at the 

 centre of the tube as near the bulb as possible, the character- 

 istic band spectrum excited by white light appears. The 

 ease with which this experiment can be performed mak^s it 

 of interest in view of the importance of resonance and band 

 spectra in connexion with the theory of the radiation of 

 electrons. 



If helium is available, a second tube may be prepared for 

 showing the transformation of the resonance into the band 

 spectrum. The tube is prepared as before, and the gas intro- 

 duced until the manometer indicates a pressure of about 5 mm., 

 after which it is sealed off from the pump. These tubes 

 keep indefinitely, require no heating, and may be set up for 

 exhibition in two or three minutes. The only advantage of 

 the quartz mercury arcs used in the present investigation lies 

 in the greater intrinsic brilliancy of the light. 



The use of glass bulbs for the study of the fluorescence 

 has been discontinued, as it has been found that tubes 

 similar to those described above give greater brilliancy and 

 far less trouble resulting from reflexion of the light of the 

 arc from the walls. It will be found advantageous to 

 wrap the end of the tube with black cloth ; this gives us a 

 dark background against which the fluorescent light stands 

 out. 



