180 TROPICAL NATURE, AND OTHER ESSAYS. 



explained. We have first to inquire what is colour, 

 and how it is produced ; what is known of the causes 

 of change of colour ; and what theory best accords with 

 the whole assemblage of facts. 



The Nature of Colour. The sensation of colour is 

 caused by vibrations or undulations of the ethereal 

 medium of different lengths and velocities. The whole 

 body of vibrations caused by the sun is termed radiation, 

 or, more commonly, rays ; and consists of sets of waves 

 which vary considerably in their dimensions and rate 

 of recurrence, but of which the middle portion only is 

 capable of exciting in us sensations of light and colour. 

 Beginning with the largest waves, which recur at the 

 longest intervals, we have first those which produce 

 heat-sensations only ; as they get smaller and recur 

 quicker, we perceive a dull red colour ; and as the waves 

 increase in rapidity and diminish in size, we get succes- 

 sively sensations of orange, yellow, green, blue, indigo, 

 and violet, all fading imperceptibly into each other. 

 Then come more invisible rays, of shorter wave-length 

 and quicker recurrence, which produce, solely or chiefly, 

 chemical effects. The red rays, which first become 

 visible, have been ascertained to recur at the rate of 

 458 millions of millions of times in a second, the length 

 of each wave being aaaooth of an inch ; while the violet 

 rays, which last remain visible, recur 727 millions of 

 millions of times per second, and have a wave-length of 

 TroTtfth of an inch. Although the waves recur at 

 different rates, they are all propagated through the ether 

 with the same velocity (192,000 miles per second) ; just 

 as different musical sounds, which are produced by 

 waves of air of different lengths and rates of recurrence, 



