G76 Prof. R. W. Wood on Resonance Spectra of 



another investigation with which I have been occupied. 

 The lamp is mounted in a large wooden box, furnished with 

 a number of apertures, so that two or three investigations 

 can be made with it simultaneously! Over one of these 

 apertures is fitted a large lantern condenser 25 cm. in 

 ifiameter, which forms an image of the mercury arc along 

 the axis of the tube containing the iodine vapour. I find it 

 advantageous to make these tubes rather larger than those 

 which 1 used at first. I now make them about 40 cm. long 

 and 4 or 5 cm. in diameter, with one end blown out into a 

 bulb 6 cm. in diameter. It is important to get this bulb as 

 clear and free from stride as possible,, and above all things 

 not to have a drop of glass at the centre. This can be ac- 

 complished by drawing out the tube sideways in closing it 

 preliminary to blowing it out, The other end of the tube is 

 drawn down to a diameter of about 5 mm,, and a thick-walled 

 constriction made close to the large tube for sealing after 

 exhaustion. Before making the constriction a few small 

 crystals of iodine are introduced into the tube, which is then 

 put in communication with a Gaede or other mercurial pump 

 and thoroughly exhausted. I usually keep the Gaede pump 

 in operation for ten minutes, as the gas passes but 

 slowly through the capillary at low pressures. The tube is 

 used end-on of course, the iodine vapour being highly 

 luminous along the axis of the tube where the image of the 

 horizontal mercury arc is focussed. The intensity can be 

 further increased by means of a strip of looking-glass placed 

 close to the wall of the tube, which reflects the rays back 

 through the vapour. In some cases I have silvered the 

 outer surface of the iodine tube along the side opposite to 

 that through which the ra) s enter. This acts as a concave 

 cylindrical reflector and focusses the rays back again at the 

 centre of the tube. If it is desired to stimulate the vapour 

 by the light of the Cooper-Hewitt arc in glass, the same 

 tube can be used mounted alongside of the lamp and as 

 close to it as possible, no lens being used. The entire mass 

 of iodine vapour within the tube becomes luminous, though 

 the intrinsic intensity is much less than with the quartz arc, 

 perhaps one third as great. The mirror should of course be 

 used in this case. 



The iodine tube was brought close up to the slit of the 

 spectrograph, no lens being used on account of the large 

 size of the luminous spot. The resonance spectra were pho- 

 tographed with a prism spectrograph furnished with three 

 very large flint-glass prisms (12 cm. high) and a Cooke 

 portrait-lens of about 1 metre focus. Higher dispersion 



— 



