172 EXPERIMENTAL PHYSIOLOGY. 



much like the pure spectral colour, only a little less saturated. 

 Change the disks, taking this time reddish-orange and yellowish- 

 green. These colours are also one of longer and one of shorter 

 wave length than yellow but they are more widely separated 

 from it in the spectrum than the first two were. When you spin 

 them you again get a yellow but this time there is a much 

 greater proportion of white in it, it is less saturated than before. 

 Replace these colours with two others still further removed 

 from yellow, although still lying one on either side of it; use 

 scarlet and blue-green. When spun together this combination 

 looks whitish or pure grey, the yellow is no longer produced. 

 Two colours such as these, which give a sensation of colourless 

 or white light when combined on the retina, are COMPLEMEN- 

 TARY COLOURS. There are innumerable such pairs, a few of 

 them being the following; orange with blue, bright yellow with 

 a more violet blue, green-yellow with violet. Try these for 

 yourself. It may be necessary to show in each case a little 

 more of one of the pair than of the other, depending on whether 

 or not they are equally bright. In experimenting on these 

 complementary colours notice that when they are arranged in 

 any proportion other than that which gives white, they produce 

 a sensation of some colour which lies between them in the 

 spectrum, just as those pairs did which were less widely sepa- 

 rated. 



Now take pairs of colours too widely separated to be com- 

 plementary, for instance red and blue, or orange and violet, 

 and see that combining them gives a sensation of purple, a 

 colour which does not exist in the spectrum at all. It is the 

 complementary colour to green. 



We have seen that a colour sensation, yellow for instance, can be 

 produced (a) by waves, acting alone, of the length of those which 

 make up the yellow part of the spectrum, or (b) by a great many 

 combinations in pairs of waves of other lengths, waves which 

 falling alone on the retina give colours which have no yellow in 

 them at all. We have also seen that two beams of different wave 

 lengths, as well as producing each a definite colour sensation when 

 it acts alone, can if combined in varying proportions give rise to 

 quite a large number of the other colours. ALL the colours, how- 



