BEYOND THE RED IN TDK SPECTRUM — BABCOCK 1 GO 



Wo pass on (() (lir riioic dclailcd sdidy ol tlio infra rod part of (!io 

 spootiuni, whore wc shall deal no lon/^or with (h(> oontinuous spoctra 

 of inoan<ioscoiit hodios, but with spootra of diiToront type which are 

 proihic'od by atoms and niolooulos actin«!; as individuals. Now atoms 

 are in one respect like opera sin<;ers — in their most strikin<]^ and 

 individual roles each roqtiires plenty of elbowrooin. Thou<rh the 

 analojry is imperfect, the continuous sijoctrum of a jrlowiufj solid 

 may he compared to the chorus, while the line spectrum of a luminous 

 <;as is more like the son<; of the prima donna, who passes from one 

 dep^ree of excitation to another, emittin*;; a rapid succession of musical 

 notes like the brilliantly colored linos which individual atoms pro- 

 duce, when far enough apart from each other, and suitably stimulated. 



"When closely bound tofrether as in a solid substance, the atoms toll 

 us the temperature of the solid, but only little of themselves. When 

 the solid is turned to tras by heatinji: it sufliciently, the atoms fjot far 

 apart and then have an opportunity to do many thinjrs otherwise 

 impossible for them. Thus each kind of atom, whether it be hyilroiren 

 or iron, sends out a characteristic spectrum consisting of numerous 

 kinds, or colors, of light, each very sharply defined. Some of these 

 " colors " are invisible but readily perceptible by instruments suitably 

 designed. Each separate color is referred to as a spectral line. The 

 supply of names for these colors soon proved inadequate and numbers 

 are now used instead. 



I>et us consider the case of hydrogen, with only some 50 or 60 

 lines in its spectrum, and think of each atom of the gas, mingled 

 with countless others all alike, as a magician having a repertoire of 

 tricks. At a given moment each atom can perform just one of its 

 various tricks, and the performance, jiractically instantaneous, is to 

 produce light of a certain color. A neighboring atom may be per- 

 forming a different trick, but it is one of the .same repertoire. The 

 combined activities of all the atoms display the whole repertoire. At 

 a later time each atom may be doing a different trick from the one 

 it formerly showed, but the combined effect is the same as before. 

 Each trick corresponds to a line in the spectrum of hj^drogon, and 

 the intensity of the line is a measure of the preference which the 

 atoms have for certain of their tricks. Every other kind of atom 

 such as oxygen, or zinc, has a characteristic repertoire. Each kind 

 may be recognized by its spectrum even under diverse conditions. 



Near the beginning of this century a great physicist. Professor 

 Planck, to whom we have already referred, formulated a theory of 

 radiation which is known as the quantum theory. This important 

 work grew out of Planck's study of the total radiation of a solid black 

 body. In 1913 Professor Bohr developed an application and exten- 

 sion of Planck's theory to the problem of line spectra emitted by iso- 



