PHYSIOLOGICAL 409 



without some acquaintance with the theory of light; but our out- 

 line must be very brief. From such a source of energy as the sun, 

 radiations rush outwards through space in waves of varying length 

 from crest to crest. The waves of X-rays, used in radiotherapy, or 

 the waves of ultra-violet light, also used to promote health, are 

 very short, while those of heat rays, and still more those used in 

 broadcasting, are increasingly long. But all have the same velocity, 

 approximately 186,300 miles per second. If the whole vast range 

 of electromagnetic radiations be compared to a series of sixty-two 

 octaves, one octave would include those rays to which our eyes are 

 sensitive, the limited range which we call visible light. It must be 

 noted, however, that the range is not quite the same for all animals 

 with eyes; thus ants, as Sir John Lubbock first showed, are sensitive 

 to ultra-violet light, which is invisible to us. 



The octave of ordinary visible light is itself made up of rays of 

 different wave-lengths; and each of these produces in our eye and 

 brain a particular colour-sensation, such as red or green. The 

 shortest are the violet rays, familiar at one end of the spectrum 

 or in the rainbow, which have a wave-length of some forty millionths 

 of a centimetre, and the longest, at the other end, are the red rays, 

 which are twice as long as the violet. A blend of what we might 

 call all the notes of the octave produces white light. 



The colours, which are the vital results of the rays of different 

 wave-lengths, may be for convenience written down in this way: 



The pairs which are here placed opposite each other in this arrange- 

 ment were named by Chevreul "complementary colours", and their 

 relation to one another is significant in understanding colouring. 

 If by some process of filtering we subtract from the complete white 

 light any one member, for example, red, then the balance will be 

 disturbed, and the light will no longer appear white, but green — 

 the colour complementary to that which was filtered out. The 

 filtering may be effected in various ways. Thus a sheet of copper will 

 remove much of the white light that falls on it, and will reflect chiefly 

 the red — whence the familiarly pleasant colour of burnished copper. 

 Yet if the sheet of copper be so thin that light can pass through, 

 then the red rays are absorbed, and the filtered light will appear 

 green. Solutions of copper salts, such as we may see in the great 

 globes in the druggist's shop-window, have a fine blue colour, 

 which is due to the fact that the ions (or wandering detached atoms) 

 of copper have the power of absorbing light at the red end of the 

 spectrum. This filtering is of fundamental biological importance, since 



