CONTEMPORARY ADVANCES IN PHYSICS 79 



ing distinct excited states; third, the excited states had energy- values 

 so low that a quantum of the violet, blue or green regions of the spec- 

 trum striking a molecule had plenty of energy to excite it and yet have 

 some left over. Subsequently one of Raman's associates (Ramdas), 

 photographing with very long exposure, detected the effect with 

 ether vapor. Carrelli and his colleagues obtained it with molecular 

 salts in aqueous solution ; ^^ and perhaps in the course of time some- 

 body may overcome the obstacles, and demonstrate the scattering of 

 light with change of frequency by free atoms of a monatomic gas. 



We will now consider some of the photographs which were made 

 by Raman, and later with improvements of technique by Wood, 

 Langer and Meggers, Brickwedde and Peters. 



Some of Raman's earliest published pictures are reproduced in 

 Fig. 2. At the top we see the spectrum of the primary light — that 

 of the mercury arc, the source of light employed, I think, in all the 

 researches thus far published. The strong lines near 4046 and 4358 

 are responsible for most of the Raman lines thus far observed by any- 

 one; other strong lines are those near 5461, and the pair at 5770 and 

 5790. (These last look much fainter in Fig. 2 than in some of the 

 others, but such variations are due to the photographic plates em- 

 ployed, and should not be heeded.) The three spectra below are, in 

 order, those of the light scattered by benzene, toluene and pentane. 

 The new lines are extremely numerous — so much so, that some care is 

 required to determine for each new line which is the primary line 

 whence it is shifted. This may be done by filtering out from the 

 primary light all but one of its strong lines. In making the photo- 

 graphs in Fig. 3, Raman and Krishnan used a filter which removed 

 from the infalling light almost all the quanta but those of the wave- 

 length 4358 (though 3650 and 4046 are still seen dimly in the spectrum, 

 the topmost one in the figure). The spectrum of the scattered light, 

 below, now shows additional lines which are certainly made of quanta 

 which originally had the wave-length 4358. 



Figs. 4 and 5 show the spectra scattered by benzene and carbon 

 tetrachloride, as photographed by Wood." The "fat" lines from 

 left to right are the unshifted lines 4046, 4358, 5461 and the aforesaid 

 doublet 5770-5790. (The rich adjacent spectrum is a "comparison" 

 spectrum of iron.) Most of the lines companioning 4358 and 5461 on 

 both sides are shifted lines. Notice, in the spectrum scattered by 



13 With salts which are completely dissociated in solution they failed, as they 

 expected, to obtain it. Possibly the effect may some day be used as a measure of 

 percentage of dissociation! 



" I am much indebted to Professor Wood for furnishing me with prints of these, 

 and to Dr. Langer for plates from which the next two figures were made. 



