10 THE FOUNDATIONS OF SCIENCE 



ideas take their rise and pass into the red of definite 

 physics. 



We may picture these relations to our eye in a 

 manner, originally suggested by Newton, and now 

 used to illustrate Maxwell's theory of colour vision. 

 On that theory, as we shall see later, there are three 

 primary colour sensations : red, green, and blue. 

 These colours are placed at the corners of a triangle, 



FIG. 2. 



and points within the triangle are tinged with com- 

 plex colours made up of the three primaries in 

 proportion to the nearness of the points to the three 

 corners. 



Now, as before, let us take red to represent physical 

 science, green biology, and blue psychology. The 

 points of the triangle represent the purest sciences : 

 red represents physics in the strict sense, green the 

 natural history of plants and animals, and blue pure 



