322 PHYSIOLOGICAL PHYSICS. [Chap. xxvi. 



E in the yellow end of the green, F in the blue end of 

 the green, G in the indigo, and H in the violet. The 

 explanation of these dark lines is the result of the 

 thought and labour of various scientific men, notably 

 Stokes, Bunsen, and Kirchhoff, but was not fully 

 offered till 1859 by Kirchhoff. 



One of the most prominent of the dark lines of the 

 solar spectrum is the D line, which, properly speaking, 

 consists of two lines, and is in the brightest part 

 of the spectrum. Frauiihofer observed that if the 

 source of light, instead of being the sun, were the 

 vapour of sodium, such as might be obtained by burn- 

 ing in the hot part of a bunsen flame some common 

 salt, and if the light from this vapour were passed 

 through a prism, a band of colours like the solar 

 spectrum was not obtained, but instead two narrow 

 bands of yellow light. If an arrangement is made for 

 obtaining the solar spectrum and the spectrum of 

 sodium side by side, one above the other, the two 

 bright yellow lines of the sodium flame are found to 

 correspond in position with the two dark lines, called 

 D, of the solar spectrum. A very minute trace of 

 sodium, even the 18 millionth part of a grain, it is 

 asserted, will give the yellow band. Similarly, 

 potassium, burnt in a bunsen flame, gives two bright 

 red lines and one violet line. Strontium gives various 

 bright red lines, and an orange line at the red side of 

 the D line. A large number of substances have been 

 examined by being volatilised before a prism, and have 

 yielded various coloured lines, the coloured lines of 

 many substances, such as sodium, hydrogen, calcium, 

 barium, iron, etc., being found identical in position with 

 dark lines of the solar spectrum. The dark lines, 

 then, of the solar spectrum indicate the absence of 

 certain rays, which, in the case of the D lines, for ex- 

 ample, the glowing vapour of sodium emits. Now, to 

 take the case of the D line, it is found that if some 



