•88 



NATURE 



lA/ay 23. 1878 



turned upwards, will have exactly undone the work done 

 by the prism with its refracting edge turned downwards. 



For the last fifteen years students have been occupied 

 in mapping the spectra of coloured light sources, and the 



Fig. 3.— Recomposition of Lght. 



result is now that every line in the spectrum of every 

 substance has been thus recorded with the utmost dili- 

 gence and care ; so that the statement that the spectrum 

 we get from the ultimate molecules of each different 



chemical substance is different, is substantiated by a most 

 overwhelming mass of evidence. 



It must be understood that in what I have stated I 

 have represented the phenomena as being much more 

 simple than they really are. It is quite true we can, by 

 toning down the molecular motion, make the spectrum of 

 each chemical element only occupy one of the spectral 

 regions as a rule ; but it is equally true that when the 

 motion is great all the regions are filled with lines as the 

 two following examples (Figs, 4 and 5) will show. 



Here, then, is the sure and certain knowledge that we 

 possess regarding the motions of molecules so far as one 

 cause of the coloured phenomena observed in Nature is 

 concerned. There is another result which has been 

 gathered in the region of the infinitely little which helps 

 us to another cause of these phenomena. 



So far we have considered these ultimate molecules in 

 a state of extreme vibration. As a matter of fact, so 

 long as we are dealing with these ultimate finenesses of 

 matter, we can still assure ourselves of the motion of the 

 molecules when their vibrations are far less virid. How 

 is this accomplished ? 



In this way. The molecules are so apt to vibrate each 

 in its own period that they will even take up vibrations 



Fic. 4, — Part of the blue region in the spectrum of a mixture of incandescent aluniinium, calcium, and iron vapours, showing ho.v the images of the slit, 



produce the appearance of bright lines. 



Fig. 5. — Pait of the blue region in the .<^psctrum of incandescent iron vapour. 



from light which is passing among them, provided always 

 that the particular wave-lengths of light in which they 

 themselves vibrate are contained in the light which they 

 receive. 



Let us imagine a case. Suppose I have at one end of 

 a room a vivid light source giving us all the possible waves 

 of light from red to violet. 



W fl © © Y © [^ 



Also suppose that I have in the middle of the room a 

 screen of molecules feebly emitting yellow light. 



What will happen? Will the light come to my eye 

 exactly as if the molecules were not there ? No ; it will 

 not. There will be a difference. What then will be the 

 difference ? 



Experimentally we find that the molecules Avhich give 

 out yellow light have kept for their own purposes— filched 

 so to speak, from the light passing through them— the 

 particular vibrations which they want to carry on their 

 own motions, and we shall have 



WO©© ® K 



as a result ; the light comes to us minus the vibrations 



which have thus been utilised, as we may put it, by the 

 screen of vapour. 



We have in fact an apparently dark space which may 

 be represented in this way 



W fl © [13 Y © ^ 



When we use a spectroscope we get the continuous 

 spectrum with a dark band across the yellow absolutely 

 identical in position with the bright band observed when 

 the molecules of the vapour of which the screen is com- 

 posed radiated light in the first instance. 



There is an experiment in the world of sound which, 

 perhaps, may render the physics of this action clearer. 

 If we go into a quiet room, where there is a piano, and 

 sing a note, and stop suddenly, we find that note echoed 

 back from the piano. If we sing another note we fiad 

 that it also is re-echoed from the piano. How is this ? 

 When we have sung a particular note we have thrown 

 the air into a particular state of vibration. One wire in 

 the piano was competent to vibrate in harmony with it. 

 It did so, and vibrating after we had finished, kept on the 

 note. 



One example of the phenomena observed when we 

 use a series of molecules as a screen, I can bring 



