170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



lated atoms, and he succeeded in accounting for the known spectrum 

 of hydrogen with astonishing accuracy and completeness. By cal- 

 culations based on this theory he showed that the known lines of the 

 hydrogen spectrum should be precisely where we find them. Further- 

 more, he predicted the exact position of other lines which the hydro- 

 gen atom should emit, and these were afterward observed precisely 

 where he said they should be. This brilliant success was followed by 

 a rapid expansion of our understanding of all line spectra and of 

 the atoms which give rise to them. 



A basic idea in Bohr's theory of spectra is this : Whenever an atom 

 produces one of its characteristic pure colors the energy thus sent 

 out in the form of radiation is exactly equal to the energy lost by the 

 atom as a result of the rearrangement of its parts. The atom is 

 thought of as having a heavy inner part or nucleus, with one or more 

 much lighter electrons bound to it by electrical forces. The analogy 

 with the structure of the solar system is obvious and to some extent 

 helpful, but it must not be carried too far. For example, the planes 

 of the planetary orbits lie within a few degrees of each other in the 

 solar system, while in the atoms this is not necessarily true. When 

 one of the outer electrons in an atom assumes a new position nearer 

 to the nucleus the atom accomplishes a definite amount of work, 

 and the energy radiated away is the exact equivalent of this work. 

 If a weight is allowed to fall from table to floor a certain amount 

 of work is done. Its equivalent is found in the heat developed, 

 the sound waves set up, and sometimes also in other ways. 



Each line in a spectrum thus signifies the existence of two states 

 of the atom in which the internal energy of the atom is different. 

 A line whose color is red arises from a transition between two states 

 not very different; a line in the ultra-violet, from a transition be- 

 tween states having a much greater difference of energy. If your 

 baby falls off a footstool he may express his annoyance by a moder- 

 ately pitched exclamation; if he falls downstairs he will emit vocal 

 sound waves at least an octave higher. The color of a spectral line, 

 accurately expressed on a frequency scale, tells exactly the energy 

 lost by the atom when it performed one of its tricks. 



The physicist measures the positions ; i. e., the " colors," to speak 

 loosely, of the spectral lines with an accuracy almost unbelievable; 

 he notes the relative intensities of the lines, their individual peculi- 

 arities such as sharpness or diffuseness, their varying behavior under 

 change of conditions surrounding the source of light, such as pres- 

 ence or absence of a magnetic field ; and gradually he learns, for each 

 kind of atom, the sj^stem of energy-states which it possesses. These 

 energy-states, or spectral terms as they are called, are less numerous 

 than the lines in the spectrum, and are far more significant of the 



