472 Prof. E. W. Wood on the 



illuminating the vapour. One of these was mounted ahove 

 the tube, and the rays, after having been brought to a focus 

 at the centre of the tube by a condensing lens of 25 cm. 

 aperture, were returned to the same focus by means of a 

 large concave mirror placed below the tube (which was at 

 the centre of curvature of the mirror). One lamp was thus 

 made to do the work of two. The other lamp was mounted 

 to one side of the tube with a similar arrangement of lens 

 and mirror. The mercury arc-lamps were about 12 cm. in 

 length, and their images formed along the axis of the iodine 

 tube by the condensing lenses and mirrors were of the same 

 dimensions. 



This arrangement appears to give us very nearly the 

 maximum amount of illumination possible, the only improve- 

 ment which I have been able to think of being an iodine 

 tube completely surrounded by a mercury arc burning in a 

 long closely-wound spiral tube of quartz. I have also had 

 constructed a glass mercury arc with the iodine tube passing 

 axially down its centre, but I have not yet succeeded in 

 getting it to work very well. With the arrangement just 

 described, I found that I could get a fully exposed photograph 

 of the resonance spectrum with a Hilger wave-length spectro- 

 graph in less than two hours, whereas in the earlier experi- 

 ments exposures varying from 18 to 24 hours were necessary. 

 The greatly increased brilliancy of the fluorescence which is 

 obtained by the use of a tube can be ascribed to the following 

 circumstance. It was found that when a large bulb was 

 used, as in the earlier experiments, if the light was returned 

 to the focus by a concave mirror placed behind the bulb, the 

 cone of fluorescent light produced by this reflected beam 

 was so faint that it contributed very little to the total 

 intensity. This resulted from ihe. absorption of the exciting 

 frequencies by the iodine vapour. It is obvious that if we 

 employ a bulb and focus the light at its centre, much of the 

 available energy is lost before the rays come to a focus. 

 This difficulty is avoided by the use of a tube, since the 

 exciting rays traverse only the small volume of iodine vapour 

 which is under observation. 



The green line and both of the yellow lines of the mercury 

 arc are capable of exciting resonance spectra which can be 

 exhibited separately. If we interpose a glass cell containing 

 a strong solution of eosine between the arc and the iodine 

 tube, the resonance spectrum excited by the two yellow lines 

 appears : this consists of a beautiful series of equidistant 

 doublets, fifteen in all (30 lines), two below and twelve above 

 the yellow exciting lines. The series of equidistant lines 



