106 A CHAPTER ON LIGHT AND COLOUR. 



hofer in 1817 rediscovered these bands independently, and, using 

 a narrower slit, was able to map their positions. The chief of them 

 are still known by the letters A^ B, C, etc. ; a, b, c, etc., by which 

 Fraunhofer referred to them. It was found that if the light of the 

 sun were made to pass through the light of the electric arc 

 coloured yellow by sodium, the dark line (or pair of lines) D was 

 marked much more plainly than in the simple solar spectrum. 



The arc tinged with sodium alone gave a bright pair of D 

 lines. These discoveries were made in 1849 t>y Foucault. 



II. — The origin of these Dark Lines in the sun's spectrum 

 was not discovered till much later. About 1850, Stokes explained 

 the phenomenon by an analogy from sound. 



We know that a tuning fork will, when sounded, excite another 

 fork of the same pitch, without touching it, and such a fork will 

 select from a composite sound the vibrations which it can itself 

 give out when sounded, thereby weakening that particular tone of 

 the composite sound. Similarly, bodies capable of vibrating at 

 such a rate as to emit light of a particular refrangibility will absorb 

 just that same kind of light from the radiations of a composite 

 character which fall on them. The absorbing body emits more 

 than it absorbs if its temperature is above that of the source of 

 light which falls on it ; but if below that temperature it absorbs 

 more than it emits, and causes a deficiency in light of that parti- 

 cular kind. 



1 2. — The spectra of different bodies are found to be charac- 

 teristic of those bodies, and are one of the most delicate tests of 

 their presence. The researches of Kirchhoff and Bunsen in i860 

 led to the establishment of the law mentioned above referring to 

 the modification of light when passed through an absorbing 

 medium, and also to the laying down of the principles that 



(i.) Incandescent solids and liquids (and, it has since been 

 found, highly compressed gases) give rise to a continuous spectrum. 



(ii.) Gases under moderate pressure give spectra of bright 

 lines and bands separated by dark spaces. 



The principles thus enunciated enable observers to detect the 

 presence of many well-known bodies, such as iron, magnesium, 

 sodium, and hydrogen in the sun and many stars and nebulae. 

 The element helium, not discovered on the earth till a couple of 



