62 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



minus green, i.e. red. For very small path difference, small in com- 

 parison to the wavelength of light, almost equal proportions of all 

 radiations will be lost and the object will appear grey. As the path 

 difference increases it will appear first white, then yellow and so forth, 

 up to red. All these colours are said to he first order colours. The point 

 at which the path difference is one wavelength for green light arbitrarily 

 marks the end of this first order. Similarly, the subsequent point at 

 which the path difference is two wavelengths for green marks the end of 

 the second order and so on. With increase in path difference, therefore, 

 a steadily changing colour is observed in an object between crossed 

 Nicols, in a regular series called Newton's Series. The colours, together 

 with the corresponding path differences, are given in Table II, and an 

 attempt has been made to reproduce the colours themselves for the 

 first order in Plate III, Fig. 1. 



TABLE II 



The first two orders of Newton's Colour Scale 

 {modified from Quincke) 



