330 NINETEENTH CENTURY. PT. HI. 



CHAPTER XXXIV. 



SCIENCE OF THE NINETEENTH CENTURY (CONTINUED). 



Spectrum Analysis and its Applications to Terrestrial and 

 Celestial Bodies Celestial Photography. 



History of Spectrum Analysis, 1800-1861. We now 



come to the history of Spectrum analysis, or the study of 

 the various coloured bands produced by different kinds of 

 light when seen through a prism. This is certainly one of 

 the most wonderful discoveries of our century, and though 

 its history is difficult, partly because it belongs to our own 

 time and is going on even now, yet we may learn something 

 about it. The first step was made, as you will remember, 

 when Newton discovered that white light is composed of 

 different coloured rays, but even he little suspected what 

 histories those rays could be made to tell. 



Discovery of Heat-rays by Sir William Herschel, 

 1800. One of the first facts which was learnt in this cen- 

 tuiy about the spectrum was, that the coloured band which 

 is seen when a ray of white light is passed through a prism 

 does not give us the whole of the dispersed ray; for there 

 are many invisible rays at both ends of the coloured part 

 which are very active, though we cannot see them. 



It had always been thought that the hottest rays must 

 be those, such as the yellow ones, which give the most light, 

 and in the year 1800 Sir William Herschel, wishing to try 



