of a Resonance Spectrum into a Band- Spectrum. 267 



the other members of the system, each one of which emits a 

 radiation of its own frequency (resonance spectrum). The 

 disturbance is not, however, communicated to the other 

 systems which remain quiet. With white light excitation 

 all of the systems are of course disturbed, and we get the 

 band-spectrum which can be perhaps regarded as a composite 

 photograph of all of the resonance spectra. Now the collisions 

 with the helium molecules appear to destroy the equilibrium 

 of the forces within the molecule, which is necessary for the 

 isolation of the electron systems, and results in a transfer of 

 energy from the excited electron not only to all the members 

 of its own system, but also to the members of the other 

 systems. Electro-negative gases are unable to accomplish 

 this since they practically destroy the fluorescence if they 

 approach near enough to the iodine molecules to affect them 

 at all. 



It is to be clearly understood that the collision with the 

 helium molecule does not excite the vibrations, but merely 

 brings about a condition which enables a transfer of energy 

 from one vibrating system of electrons to the other systems 

 to take place. The total amount of light emitted by the 

 iodine vapour in vacuo and in helium at 2 mm. pressure is 

 the same : in the former case it is concentrated in the 

 15 resonance spectrum lines, in the latter it is distributed 

 among the hundreds of lines which make up rhe band- 

 spectrum. The two spectra can be beautifully shown with 

 the smallest direct-vision spectroscope. It is best to use a 

 bulb 15 or 20 cm. in diameter, and to illuminate the vapour 

 with the light issuing from the side of a quartz mercury 

 lamp, brought to a focus at the centre of the bulb with a 

 large lantern condenser. In this way we obtain a broad 

 sheet of illuminated vapour, which when viewed from the 

 side has a considerable intensity. It is obvious that one 

 effect of the collision with the helium molecule is to reduce 

 the amount of resonance radiation (unchanged wave-length) 

 in proportion to the total radiation. This explains perfectly 

 why the helium reduces the intensity of the green portion of 

 the fluorescent spectrum excited by white light more rapidly 

 than the red, as we have shown in a previous paper. The 

 fluorescence is excited chiefly by the green rays, the fluo- 

 rescence spectrum extending from wave-length 500 to the 

 extreme red. The region of the spectrum chiefly operative 

 in exciting the fluorescence is from 500 to 580. Orange and 

 red light is wholly inoperative. 



In the complete band-spectrum excited with white light 

 we may therefore consider the green portion as made up 



