enormous absorbing or emitting power, so that only a small part of the matter in a given region is concerned with the light and heat emitted or absorbed in the region. Light beams that are sufficiently intense to study are apparently emitted by a very large number of light centers and for this reason it has been found impossible to isolate individual centers; and even if this were possible it may be that the life of these centers is so short that even the isolation of centers would not permit their being studied. Then again, light emission is usually accompanied by many intricate phenomena such as ionization and chemical reactions and this adds to the complexity of the problem. When we consider our profound ignorance respecting even the nature of chemical forces, the constitution of the molecule and atom, the nature of the electric and magnetic fields and even the nature of light itself, it is not at all remarkable that little definite and certain knowledge has been obtained concerning the nature of light centers.
Some Methods of Approaching the Problem
There are several avenues of approaching the problem of the nature and constitution of light centers that seem to be extremely inviting.
1. At the present time a wonderful field is being opened concerning the dynamics of chemical reactions. As chemical reactions are intimately related to heat and light effects, the discoveries in this field are bound to give a great deal of information concerning light centers.
2. A study of the far infrared promises to break the gap between electromagnetic waves and radiant heat and light centers will probably be found to consist of molecular systems vibrating in a way similar to that of the sources of electromagnetic waves. At the present time we can compare light and heat centers with more or less well-known aggregates of matter and make as many identifications as possible.
3. The separation of complex line and band spectra into series of related lines or bands promises to give us a great deal of information ultimately as to the nature of the vibrating centers, although at present the problem is so complex that no one has been able to devise any mechanism or structure that is adequate to explain the known phenomena. The theory of Ritz has been one of the most successful so far advanced.
4. The Zeeman effect obtained by placing the heat and light centers in a magnetic field is important. This effect indicates that many of the centers of spectral lines consists of negative electrons.
5. The Humphreys-Mohler pressure shift of spectrum lines, the Döppler shift of lines and bands emitted by moving centers as studied by Stark and others, are also very important.
Possible Structure of Light and Heat Centers
One may picture light and heat centers as consisting in part as follows:
