en. XX 



THE DISPERSION OF LIGHT. 



i6s 



Descartes had gone farther, and had pointed out that a ray 

 of light seen through a clear, angular, polished piece of glass, 



Fig. 28. 



called a prism (see Fig. 28), is spread 

 out into colours exactly like the rain- 

 bow; but no one had yet been able 

 to say what was the cause of these dif- 

 ferent tints. Newton was the first to 

 work this out in his usual accurate and painstaking way. 

 He tells us that in 1666 he 'procured a triangular glass 

 prism, to try therewith the celebrated phenomena of 

 colours,' and in the very first experiment he was struck by a 

 very curious Jact. He had made a round hole f (Fig. 29), 



Fig. 29. 



Newton's first Experiment on Dispersion of Light. 



D E, Window shutter. 



F, Round hole in it. A B c. Glass prism, 

 which the spectrum was thrown. 



M N. Wall on 



about one-third of an inch broad, in the window-shutter, 

 D E, of a dark room, and placed close to it a glass prism, 

 A B c, so as to refract the sun-light upwards towards the 

 opposite wall of the room, m n, making the line of colours 

 (red, orange, yellow, green, blue, indigo, and violet), which 

 Descartes had pointed out, and which Newton called a 

 spectrum^ from specto^ I behold. 



While he was watching and admiring the beautiful 

 colours, the thought struck him that it was curious the 

 spectrum should be long instead of round. The rays cf 



