213 
be regarded rather as conditions which modify, than 
as causes which produce the phenomena : and hence 
the mechanical properties of bodies would, in such 
actions, be considered subordinate to those which a- 
rise from their chemical constitution. 
490. But whatever hypothesis may be adopted 
concerning the cause of refraction and reflexion, 
the certainty that these effects are produced, and that 
to them the colours of natural bodies are attributable, 
will not be disputed. It only remains, therefore, 
that we apply our knowledge of these subjects to the 
explanation of those colours with which nature has 
adorned and embellished the vegetable world. 
491. The green colour which plants exhibit may 
be either simple or compound. Newton remarks, 
that a leek appears most resplendent when viewed 
by green light, and next when seen by the blue and 
yellow rays which compound a green *. In the 
opinion of M Bertholiet, the green colour of plants 
is produced by simple and hoinogeneal light, and 
not by a compound of the yellow and blue rays t ; 
but M. Haiiy asserts, that, if a slip of a green plant 
be laid on yellow paper, and held between the light 
and the eye, and the paper be then agitated so as to 
aid the sensation, the green slip will appear more or 
less blue, which proves, says he, that the green 
colour is a compound of yellow and blue, and not a 
.simple colour +. 
* Optics, B. i. part 2. prop. 10. 
t Elem. de la Teinture, torn. i. p. 12. 
: Traite de Phys. t. ii. p. 64. 2d edit. 
