General Theory of Radiation. 383 



the screen, the image, instead of being the shape of the hole it came 

 through, is greatly elongated in one direction, forming a band, and is 

 differently colored in the different parts. One end of this colored band 

 is red, the other end violet. The red end is nearly in a straight line 

 produced from the slit through the prism to the screen, this ray being 

 bent but little in passing through the prism, while all the other colored 

 rays are bent more than it, in the following order : orange, yellow, 

 green, blue, indigo and violet ; the last being deflected the most. This 

 colored band is called a spectrum. The spectrum of sunlight, when it 

 is obtained by passing the beam of light through a very narrow slit in- 

 stead of a large opening, is found to be crossed by numerous dark lines. 

 These were mapped on the spectrum, and some of them designated by 

 letters by Fraunhofer, in whose honor they are called Fraunhofer's Lines. 



Table showing Wave Lengths of various Colored Light, No.' of Waves per inch, and No. 

 of Undulations per second : 



It has been found that these dark lines are in consequence of portions 

 of sunlight being intercepted or absorbed by incandescent gases floating 

 in the atmosphere of the sun. If a bit of sodium or salt be vaporized 

 in a Bunsen burner and then a ray from an electric lamp be sent through 

 it and through a prism, a continuous spectrum will be formed having all 

 the colors from red to violet. But across the orange there will be a 

 dark line in the position marked D on Fraunhofer's charts. The D line 

 of the solar spectrum is thus found to be caused by the absorption of 

 some of the orange light from the body of the sun by incandescent sod- 

 ium vapor suspended in its atmosphere. In like manner the line marked 

 F on the spectrum is found to be due to the interposition of hydrogen. 

 Other lines are caused by iron, some ( H and others) by calcium, others 

 by nickel, cobalt, manganese, potassium, &c. Twenty-two of the ele- 

 ments existing on earth have been identified in this manner, as also 



