496 Dr. Arthur Schuster [Jan. 28, 



a prism and project them, as I do now, on a screen, you see a con- 

 tinuous band of light. This fact we express by saying that the 

 spectrum of the carbon poles in the electric lamp is a continuous one. 

 You see side by side the different colours known to you by the familiar 

 but incorrect name of " the rainbow colours " ; and the experiment 

 teaches you that the carbon pole of the electric lamp sends out rays 

 in which all wave lengths which produce a luminous sensation are 

 represented. 



But if now I introduce into the electric arc a small piece of a 

 volatile metal you see no longer a continuous band of light. The band 

 is broken up into different parts. Narrow bands or lines of different 

 colours are separated by a space sometimes black, sometimes slightly 

 luminous. The metal has been converted into vapour by the great 

 heat of the electric current, and the vibrations of its molecules take 

 place in distinct periods, so that the waves emanating from it have 

 certain definite lengths. If the molecule could only send out one 

 particular kind of waves, I should in its spectrum only see one single 

 line. We know of no body which does so, though we know of several in 

 which the jDOSsible periods of vibration are comparatively few ; the 

 spectrum of these will therefore contain a few lines only. Thus we 

 have two different kinds of spectra, continuous spectra and line spectra. 

 But there is a certain kind intermediate in appearance between these 

 two. The spectra of " fluted bands," as they are called, appear, when 

 seen in spectroscopes of small dispersive powers, as made up of bands, 

 which have a sharp boundary on one side and gradually fade away 

 on the other. When seen with more powerful instruments each band 

 seems to be made up of a number of lines of nearly equal intensity 

 which gradually come nearer and nearer together as the sharp edge 

 is approached. This sharp edge is generally only the place where 

 the lines are ruled so closely that we can no longer distinguish the 

 individual components. The edge is sometimes towards the red, 

 sometimes towards the violet end of the spectrum. Occasionally, 

 however, the fluted bands do not show any sharp edge whatever, but 

 are simply made up of a series of lines which are, roughly speaking, 

 equidistant. No one who has seen a spectrum of fluted bands can 

 ever fail to distinguish it from the other types of spectra which I have 

 described. 



What, then, is the cause for the existence of these different types ? 

 The first editions of text-books in which our science was discussed 

 stated that a solid or liquid body gave a continuous sijectrum, while a 

 gaseous body had a spectrum of lines ; the spectra of bands were not 

 mentioned. The more recent editions give a few exceptions to this 

 rule, and the editions which have not appeared yet, will — so I hope, at 

 least — tell you that the state of aggregation of a body does not 

 directly affect the nature of the spectrum. The important point is 

 not whether a body is solid, liquid, and gaseous, but how many 

 atoms are bound together in a molecule, and how they are bound 

 together. This is one of the teachings of modern spectroscopy. A 



