224 SCIENCE PROGRESS 



We might at first be inclined to suppose that these three 

 characteristics are independent of one another, and that one 

 kind of radiation can differ from another in any or all of them. 

 There are two facts, however, which relate them in such a 

 way that a wave is completely known when a single quantity 

 is given. In the first place, we see that the product of the wave- 

 length and the frequency gives the total length of the wave- 

 train passing a given point per second — or, in other words, 

 the velocity of the waves — so that V = nX. Secondly, it has 

 been found that, in the free ether, all waves travel with the 

 same velocity, so that n\ is a constant — or, in other words, 

 the frequency is inversely proportional to the wave-length. 

 It follows that we have only to state the frequency or the wave- 

 length of any kind of radiation in order to define it completely. 

 We can regard etherial radiation, therefore, as a process of 

 transfer of energy, the energy being stored in a number of 

 wave-trains of different frequency or wave-length, just as a load 

 of furniture can be conveyed from one place to another in vans 

 of different sizes. 



It is in the separation of the different kinds of radiation, 

 and their subsequent examination, that spectroscopy finds 

 its metier. Spectroscopists now employ more than one means 

 of separation, but it will be sufficient for our purpose to consider 

 the historic method by which Newton first analysed sunlight. 

 We have said that all waves travel with the same velocity in 

 the free ether. The conditions are different, however, when 

 the ether is not free but is in the interior of a transparent 

 material body. In these circumstances, the velocity of all the 

 waves is reduced, and the shorter the wave-length, the greater 

 the reduction. Let us suppose, therefore, that a composite 

 beam of radiation is made to pass through a block of glass. 

 The reduction of velocity will declare itself by a bending, or 

 refraction, of the radiation, the shorter waves being bent more 

 than the longer ones. The directions of the different waves 

 in the glass will therefore depend on the wave-length, and, on 

 emerging from the glass, the beam will have been analysed 

 into its components. It is just as though our furniture vans, 

 which at first travel together along the open country roads, 

 enter the busy street of a town. The larger vans are able to 

 force a passage through, while the smaller ones suffer more delay 

 from the traffic, and fall behind. On reaching the country 

 beyond, they will have been separated from their companions. 

 In practical work, it is customary to cut the analysing substance 

 into the shape-of a triangular prism, and to allow the radiation 

 to enter it after passing through a narrow slit parallel to the 

 refracting edge of the prism. In this way, the analysed beam, 

 or " spectrum," consists of a number of images of the slit, 



