CH. XX 



THE DISPERSION OF LIGHT. 



165 



Glass Prism. 



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

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

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



out into colours exactly like the rain- 

 bow ; but no one had yet been able At 

 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 fact. 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. ABC, Glass prism. M N, Wall on 



which the spectrum was thrown. 



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 < f 



