410 



Dr. R. A. Houstoun on a 



part remains straight and the relative spacing of the points 

 on it is not altered, but that the curvature o£ the curved 

 part alters, and the curve still retains the same rough 

 resemblance to a parabola. 



Fig. 2. 



For the purpose of Newton's colour diagram any three 

 colours may be taken as fundamental colours. The advantage 

 of Maxwell's three standards is that they are convenient for 

 experimental purposes. The upholders of the Young- 

 Helmholtz theory, however, maintain that a certain three 

 colours are distinguished from all others in being perceived 

 each by a single sensation. There are thus three unique 

 points on the plane. It is clear that the triangle joining 

 these points must include all actual colours, because when a 

 colour is expressed in terms of fundamental sensations, there 



