14 Larmor, Physical Aspect of the Atoviic Theory. 



length to periodic time, must be altered, precisely in the 

 manner that optical observation confirms. 



The atomic constitution of matter is thus involved, in 

 a highly refined manner, in the group of phenomena 

 connected with the chromatic dispersion and absorption 

 of radiation. But to gain conditions of ideal simplicity, 

 we must attend to the case of gases where each molecule 

 is isolated and free from encounter with others during the 

 period required for thousands of its optical vibrations, 

 where in fact it can be treated as vibrating free. The 

 sympathetic vibration and resulting absorption along the 

 spectrum, in liquid and solid bodies, are influenced in 

 complex ways by the transient combinations of the 

 molecules into groups : though the general relations 

 elucidated by experiment between specific mean 

 refraction and chemical structure suggest something 

 often approximating to mere simple aggregation. 



The Function of Conceptual Models. 



In the case of every successful scientific theory, the 

 time must come when its first easy triumphs become 

 exhausted, and what prominently confront the investi- 

 gator are its outstanding defects and difficulties. When 

 this stage arrives, one way of saving appearances is to 

 purify the theory by banishing all terms which have an 

 illustrative or analogical connotation, expressing its 

 verified relations alone by new words which represent 

 simply general types of mathematical quantity — vectors, 

 scalars, rotors, etc. When such a state of crystallisation 

 has fully set in, further progress in general views is hardly 

 to be hoped for — the sources of invention are dried. up — 

 though details in such a restricted abstract scheme will 

 continue to be filled in, while new phenomena will 

 probably suggest arbitrary unexplained additions to its 

 content. 



