PDF - Wallace Online

v COLOURS OF ANIMALS 355radiation, or, more commonly, rays ;and consists of sets ofwaves which vary considerably in their dimensions and rateof recurrence, but of which the middle portion only is capableof exciting in us sensations of light and colour. Beginningwith the largest waves, which recur at the longest intervals,we have first those which produce heat-sensations only ;asthey get smaller and recur quicker, we perceive a dull redcolour ;and as the waves increase in rapidity and diminish insize, we get successively sensations of orange, yellow, green,blue, indigo, and violet, all fading imperceptibly into eachother. Then come more invisible rays, of shorter wavelengthand quicker recurrence, which produce, solely orchiefly, chemical effects. The red rays, which first becomevisible, have been ascertained to recur at the rate of 458millions of millions times in a second, the length of each wave-being ?6 ^ 60 of an inch ;while the violet rays, which lastremain visible, recur 727 millions of millions times per second,and have a wave-length of a ^ lfl of an inch. Although thewaves recur at different rates, they are all propagated throughthe ether with the same velocity (192,000 miles per second) ;just as different musical sounds, which are produced bywaves of air of different lengths and rates of recurrence, travelat the same speed, so that a tune played several hundredyards off reaches the ear in correct time. There are, therefore,an almost infinite number of different colour-producingundulations, and these may be combined in an almost infinitevariety of ways, so as to excite in us the sensation of all thevaried colours and tints we are capable of perceiving. Whenall the different kinds of rays reach us in the proportion inwhich they exist in the light of the sun, they produce thesensation of white.If the rays which excite the sensation ofany one colour are prevented from reaching us, the remainingrays in combination produce a sensation of colour often veryfar removed from white. Thus green rays being abstractedleave purple light; blue, orange-red light; violet, yellowishgreenlight, and so on. These pairs are termed complementarycolours. And if portions of differently colouredlights are abstracted in various degrees, we have produced allthose infinite gradations of colours, and all those varied tintsand hues which are of such use to us in distinguishing