THE TEACHINGS OF MODERN SPECTROSCOPY. 471 



overlap, but produce their impressions side by side of each other, and 

 their examination and investigation is therefore rendered possible. 



The elements of spectroscopy will be familiar to most of you, but 

 you will forgive me if I briefly allude to some points, which, though 

 well known, are of special importance in the considerations which I 

 wish to bring before you to-night. 



When a body is sufficiently hot it becomes luminous, or, to speak 

 in scientific language, the vibrations which are capable of producing a 

 luminous sensation on our retina are increased in intensity as the tem- 

 perature is raised, until they produce such a sensation. By means of 

 a strong electric current I can in the electric lamp raise a piece of car- 

 bon to a high temperature. When looked at with the unaided eye it 

 seems whitehot, but, when I send the rays through a prism and pro- 

 ject them, as I do now, on a screen, you see a continuous 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 dif- 

 ferent colors known to you by the familiar but incorrect name of " the 

 rainbow-colors " ; 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 vola- 

 tile metal, you see no longer a continuous band of light. The band is 

 broken up into different parts. Narrow bands or lines of different col- 

 ors are separated by a space sometimes black, sometimes slightly lumi- 

 nous. The metal has been converted into vapor 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 possible 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 disper-sive 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 indi- 

 vidual components. The edge is sometimes toward the red, some- 

 times toward the violet, end of the spectrum. Occasionally, however^ 

 the fluted bands do not show any sharp edge whatever, but are sim- 

 ply made up of a series of lines which are, roughly speaking, equi- 



