BEYOND THE RED IN THE SPECTRUM — BABCOCK 171 



nature of the utoiu. TliUiJ in the spectrum of neon some 900 lines 

 represent transitions between only liG spectial terms. 



These terms or energy -states themselves correspond to dillerent 

 arrangements of the electrons which make up the outer part of 

 the atom. Thus a knowledge of the terms, which is derived from 

 the observed spectral lines, brings us some definite idea of the 

 mechanics of the atom. 



The })eriodic table was long ago recognized by chemists as express- 

 ing some fundamental relations between the elements. By compari- 

 son of the spectra of elements adjacent in this table, the physicist is 

 now able to trace, in part at least, the gradual increase in mechanical 

 complexity of the atoms with their increase of weight. The most 

 fundamental physical and chemical relationships are thus clarified, 

 and man's mastery of his environment is delinilely increased. 



This digression from the infra-red part of the spectrum has been 

 necessary in order to show in some fashion what the spectroscopist 

 is trying to do, regardless of the spectral region in which he works. 

 As we have seen, the object is to obtain, by a study of its spectral 

 lines, as complete a picture as possible of the system of terms 

 associated with each kind of atom and from these to derive the 

 structure of the atom. To do this a large range of wave-lengths 

 must be examined; it is not sufficient to measure just the blue part 

 of the iron spectrum and attempt to find all the terms from the lines 

 shown there, for terms may exist whose combinations give no blue 

 lines at all. Even the entire visible spectrum is inadequate, and it 

 becomes necessary to study the ultra-violet and the infra-red in order 

 to fill in all the details. 



These are some of the reasons which justify the activities of the 

 spectroscopist and warrant his efforts to extend the photographic 

 study of atomic spectra into the infra-red. Two other promising 

 fields of study also invite the more detailed exploration of the region 

 of longer wave lengths, namely the sj^ectra which are characteristic 

 of molecules as distinguished from atoms, and the solar spectrum. 

 The complexity of molecules as compared to atoms is obvious, since a 

 molecule is a cluster of two or more atoms which have approached 

 each other close enough to form a partnership. Indeed the struc- 

 tural complexity is reflected in the spectra which molecules give. 

 Such spectra present many individual lines corresponding to a single 

 electronic transition in the molecule, because the atoms which make 

 up the molecule are free to rotate and to vibrate with respect to the 

 center of mass, and each of these possibilities is utilized every time 

 the electrons are rearranged. Thus the sjicctra of molecules have 

 furnished information (;f the utmost importance to the chemist as 

 well as to the physicist. For example, accurate determinations of 



