Sir David Brewster on the Colours of Natural Bodies. 469 
Newton this theory has been generally received and admired: 
In our own day it has been ingeniously defended, and beauti- 
fully illustrated, by M. Biot; and, with few exceptions, it has 
been adopted by most of the distinguished philosophers of the 
present age. 
The author of this theory has presented it under the two 
following propositions, one of which states the general cause 
of the phznomena, and the other the particular constitution 
of natural bodies on which their colours depend. 
1. “ Kvery body reflects the rays of its own colour more 
copiously than the rest, and from their excess or predomi- 
nance in the reflected light, has its colour. 
2. “ The transparent parts of bodies, according to their se- 
veral 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 analyse 
one leading phenomenon of colour, and to apply this analysis 
as an experimentum 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 se- 
lection 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 di- 
stinctly 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 
vegetables seems to be, partly by reason of the intenseness of 
their colours, 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 earthy 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 gra- 
