2 The Outline of Science 



the light may he hroken up into waves of different lengths. What 

 call light is a series of minute waves in ether, and these waves 

 are measuring them from crest to crest, so to say of various 

 lengths. Kach wave-length corresponds to a colour of the rain- 

 bow. The shortest waves give us a sensation of violet colour, 

 and the largest waves cause a sensation of red. The rainbow, in 

 fact, is a sort of natural spectroscope. (The meaning of the 

 rainbow is that the moisture-laden air has sorted out these waves, 

 in the sun's light, according to their length.) Now the simplest 

 form of spectroscope is a glass prism a triangular-shaped piece 

 of glass. If white light (sunlight, for example) passes through a 

 glass prism, we see a series of rainbow- tinted colours. Anyone 

 can notice this effect when sunlight is shining through any kind 

 of cut glass the stopper of a wine decanter, for instance. If, 

 instead of catching with the eye the coloured lights as they 

 emerge from the glass prism, we allow them to fall on a screen, 

 wfc shall find that they pass, by continuous gradations, from red 

 at the one end of the screen, through orange, yellow, green, blue, 

 and indigo, to violet at the other end. In other words, what we 

 call white lir/ht is composed of rays of these several colours. They 

 go to make up the effect which we call white. And now just as 

 water can he split up into its two elements, oxygen and hydro- 

 gen, so sunlight can be broken up into its primary colours, which 

 are those we have just mentioned. 



This range of colours, produced by the spectroscope, we call 

 the solar spectrum, and these are, from the spectroscopic point of 

 view, primary colours. Each shade of colour has its definite posi- 

 tion in the spectrum. That is to say, the light of each shade of 

 colour i corresponding to its wave-length) is reflected through a 

 certain fixed angle on passing through the glass prism. Every 

 possible kind of light has its definite position, and is denoted by a 

 number which gives the wave-length of the vibrations constituting 

 that particular kind of light. 



Now, other kinds of light besides sunlight can be analysed. 



