Sir DAVID BREWSTER on the Colours of Natural Bodies. 539 



2. " The transparent parts of bodies, according to their seve- 

 ral sizes, reflect rays of one colour, and transmit those of another, 

 on the same ground, that thin plates or bubbles do reflect or 

 transmit those rays." 



In estimating the truth of the theory which is contained in 

 these two propositions, I do not intend to enter into any ex- 

 amination of the postulates, facts, and reasonings, on which it is 

 founded. The object of the following paper is to analyze one 

 leading phenomenon of colour, and to apply this analysis as an 

 eacperimentum crucis, in determining the true origin of all colours 

 similarly produced. 



The colour which I have chosen for this purpose is the Green 

 colour of the vegetable world, and I have made this selection for 

 the following reasons : 



1 . The green colour of plants is the one most prevalent in 

 nature. 



2. It is the colour of which Sir ISAAC NEWTON has most dis- 

 tinctly described the nature and composition. 



3. Its true composition is almost identically the same in all 

 the variety of plants in which it appears. 



Sir ISAAC NEWTON has described this colour in the following 

 manner : 



" There may be good greens of the fourth order, but the 

 purest are of the third. And of this order the green of all vege- 

 tables seems to be, partly by reason of the intenseness of their co- 

 lours, and partly because, when they wither, some of them turn to 

 a greenish-yellow, and others to a more perfect yellow or orange, or 

 perhaps to red, passing first through all the aforesaid intermediate 

 colours. Which changes seem to be effected by the exhaling 

 of the moisture which may leave the tinging corpuscles more 

 dense, and something augmented by the accretion of the oily 

 and earth}' part of that moisture. Now the green, without doubt, 

 is of the same order with those colours into which it changeth, 

 because the changes are gradual, and those colours, though usu- 



VOL. XII. PART II. 3 Z 



