PHOTONS AND ELECTRONS 31 



is to be used to produce a resonance-effect or some other which only the 

 wave-length Xo can have. The effect is known as "self-reversal." 



The electric spark is a source of light distinguished by high voltage and 

 high current density, qualities bought at the sacrifice of duration. Its 

 spectrum displays lines of the gas through which it passes and lines of the 

 elements to be found in the electrodes between which it springs. Some of 

 these are not found in the ordinary arc spectra of these elements and 

 are known as "spark lines." Shortly after the advent of Bohr's atom 

 model, it was reaUzed that these are spectrum lines of atoms which have 

 lost one or more of their normal quota of electrons, the transitions 

 responsible for these Unes taking place among the electrons remaining. 

 Conditions in the spark are evidently more favorable than those in the arc 

 for maintaining a perceptible percentage of such ionized atoms in excited 

 states. Of late years the study of these lines has grown enormously, 

 partly because of the development of the technique of spectroscopic 

 research in the far ultra-violet (Schumann and Lyman regions) where 

 most of them — though not, of course, the early-discovered spark lines — 

 are to be found ; partly because they can now be produced in steady glow 

 discharges of moderately high voltages with independently heated 

 cathodes. It is even possible to map the spectra of certain atoms 

 (tellurium for instance) which lack no fewer than eight of their normal 

 quota of electrons ! There are, for instance, no fewer than seven known 

 spectra of tellurium, designated in their order by the symbols Te I 

 {i.e., tellurium I), Te II, . , . Te VII, in which the Roman numeral 

 stands for one more than the number of missing electrons. There are 

 consequently also no fewer than seven spectra — all of them known, and 

 designated as palladium I, silver II, cadmium III, indium IV, tin V, 

 antimony VI, tellurium VII — corresponding to various atoms all with the 

 same number of electrons, i.e., 46, and nuclear charges running from 

 46 to 52, inclusive. The detailed comparisons between all of these 

 spectra are of great importance for atomic theory. 



PRODUCTION OF LIGHT OF THE OPTICAL SPECTRUM : OPTICAL MEANS 



When a beam of hght of frequency v falls upon a cool, rarefied, and 

 unexcited gas, the results depend upon whether or not v coincides with the 

 frequency of one of the absorption lines. If there is no coincidence, we 

 observe the phenomenon of scattering — with or without change of fre- 

 quency — which will be the topic of subsequent sections. If there is 

 coincidence, some of the photons will be absorbed by atoms, putting 

 these into excited states. From these excited states the atoms may 

 spontaneously return to normal, emitting photons exactly like those which 

 they absorbed. This is the simplest of all cases of the production of light 

 by light, the new kind being identical with the old. The atoms may. 



