74 THEORETICAL BIOLOGY 



the mixed colours formed from them. If we put down the 

 red point on paper, we can draw a straight Hne to the yellow 

 point, and on this line we put in the red-yellow tints. We 

 cannot carry the straight line from the red any further, for 

 here we are dealing no longer with red-yellow colours, but 

 with colours that are a mixture of yellow and green. To 

 express the difference, we must give the line a new direction. 

 So we shall place the green point below the yellow and connect 

 these two by means of a line representing the colours that 

 are a mixture of yellow and green. When we reach the green 

 point, we give the line another direction, towards the blue 

 point which is placed below the red. That is to say, it appears 

 that the colours of the spectrum following blue from violet 

 have obviously an admixture of red. So we may bring back 

 the line from the blue point straight to the red. In this way 

 we get a square, on which we may set equilateral pyramids, 

 one above and one below ; this gives a hexagon. The apex 

 of the upper pyramid shall be white and that of the lower one 

 black. While all the angles of the hexagon carry pure colours, 

 on each of the edges will lie colours arising from a mixture of 

 two. The faces of the hexagon, on the other hand, will 

 display colours formed by mixture of three colours, which 

 radiate outwards, in diminishing strength, from the three 

 angles surrounding each face. If we make a transverse section 

 through a face, we can always tell at what level the single 

 colours lie above one another, in order to produce in this 

 way all possible mixed colours. 



A colour hexagon of this kind reproduces in visible form, 

 even if imperfectly, the laws obtaining within the circle of the 

 qualities of colour. 



THE PRINCIPLE OF COMPARISON 



If we ask how it was possible to transfer into the realm 

 of space the laws regulating a form of a relationship that is 



