162 



SE VENTEENTH CENTUR Y. 



FT. III. 



exactly like the rainbow ; but no one had yet been able to 

 say what was the cause of these different tints. Newton 

 was the first to work this out in his 

 usual accurate and painstaking way. 



He tells us that in 1666 he 'pro- 

 cured 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. 30), 



FIG 30. 



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 F, 

 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 of 

 light come from the sun, which is round, therefore if they 

 were all bent or refracted equally, there ought to be a 

 round spot upon the wall ; instead of which it was long 

 with rounded ends, like a sun drawn out lengthways. What 



