6 THE CHEMISTRY OF THE SUN. [CHAP. 



become of classic interest. He allowed a beam of sunlight to 

 enter a dark room through a hole in a shutter, and strike on 

 a prism so placed that its edges were horizontal and also 

 that the beam entered it obliquely by one of its surfaces. 

 He then received the light on a screen, and saw, as Kepler 

 had seen before him, a band of colours arranged in the 



FIG. 2. Decomposition of light by the prism. Unequal refrangibility of the 

 colours of the spectrum. 



same order as those of the rainbow. In this experiment, 

 if the base of the prism be upwards, the lowest colour will 

 be red, next above orange, passing by imperceptible gradations 

 to yellow, and afterwards to green, which then passes through 

 the shades of greenish blue till it becomes a pure blue, then 

 indigo, finally ending with a violet colour. The transition 



