CONTEMPORARY ADVANCES IN PHYSICS 



77 



countervailing their reciprocal repulsion, are shielded from the outer 

 world, presumably by other electrons lying still farther outward from 

 the nuclei ; and partly because their energy- values are so conveniently 

 low. This latter point can best be illustrated with an example. 

 To perceive a relatively feeble optical effect, or one which is expected 

 to be feeble, it is best to produce it in the visible spectrum — not merely 

 in order to observe it with the eye ; the major reasons are rather, that in 



Fig. 2 — At top, spectrum of primary light (mercury arc) ; below, spectra of light 

 scattered by benzene, toluene, pentane respectively. (C. V. Raman; Indian Journal 

 of Physics.) 



going away from the visible spectrum-range in one direction we find 

 the photographic plates becoming rapidly less sensitive, while in the 

 other direction the transmission of the rays through matter grows 

 steadily worse. Suppose then that one tries to produce the Raman 

 effect by light near the high-frequency limit of the visible — say about 

 4000 A., where the quantum-energy is about 3 equivalent volts. If 



