205 
to have any one property in common, or to produce 
any one effect in common, we must think ourselves 
entitled to conclude, that they exert but one com- 
mon action in their chemical operation on the saline 
compounds of plants. 
479. BUT granting that Light acts chemically on 
the juices of plants, in the manner that has now 
been stated, and changes their composition, so as 
variously to affect their colour, yet, as this action is 
confined to the invisible rays, the actual production 
of colour cannot be ascribed wholly to chemical 
agency ; and the question, therefore, still returns, 
How do plants, or the juices of plants, act on the 
visible or colorific rays of light, so as to present that 
rich variety of colours, which they actually exhibit 
to our view ? To give some insight into this matter, 
it is necessary, as Sir Isaac Newton observes, to un- 
derstand not only the nature of bodies, but the na- 
ture of light ; for both, he adds, must be under- 
stood, before the reasons of their actions upon one 
another can be known *. 
480. Having, therefore, as he conceived, esta- 
blished the fact, that " bodies reflect and refract 
light nearly in proportion to their densities," he 
proceeded to consider those properties of light which 
occasion it to be thus affected. These properties 
he supposed to consist in certain dispositions of the 
* Optics, B. ii. part 3. prop. 10. 
