NOTES ON NEBULA. 135 



created, and has already conducted to many profoundly 

 interesting discoveries in the most varied branches of 

 science. 



In the application of the spectroscopic method it is not 

 indispensably necessary that we actually have a fragment 

 of the substance ; all we require is a beam of light which 

 that substance can be made to yield when heated to a suf- 

 ficiently high temperature. No doubt this statement 

 should receive some more precise qualifications, but for 

 our present purpose it will indicate the nature of spectrum 

 analysis with sufficient accuracy. 



To begin with a simple case, the colour of a light will 

 often afford an indication of its character. Thus the red 

 light seen in displays of fireworks is due to the presence 

 of the element strontium. The ghastly yellow hue pro- 

 duced by burning common salt with spirits of wine is 

 equally characteristic of sodium. Rarely, however, in 

 nature is a simple unmixed light presented to us, as it is 

 no doubt in the two cases I have mentioned. Suppose 

 that a number of distinct hues are blended into a single 

 beam, we could hardly expect to recognise the combina- 

 tion they produce. We must have some method for disen- 

 tangling the several ingredients so that they can be tested 

 separately. 



The spectroscope gives the means of effecting the required 

 decomposition. A beam of light is passed through a 

 wedge-shaped piece of glass called a prism, or more fre- 

 quently through a whole series of prisms. If the light 

 under examination be a sunbeam, then the prism unfolds 

 a beautiful series of hues : the red, orange, yellow, green, 

 blue, indigo, and violet forming all the colours of the rain- 



